This has been much discussed over the decades since the war and there are various views on this. Even if historical bias could be left aside, we are still left with the problem of measuring or quantifying such factors as experience, determination, planning, weaponry, training and unity of purpose against each other. For example, at what point do determination and / or unity of purpose outweigh better or more weapons and / or soldiers?
However, one can summarize the factors raised, and most of these ultimately favoured the Israelis. How much weight can be given to each is never likely to be agreed upon, but we can at least reasonably conclude that no single factor accounted for the Arab failure in the war; rather, it was a combination of factors, with lack of unity and failure to commit more of their forces perhaps being the most important reasons.
First, recent historians have challenged the idea the conflict was a case of 'David vs. Goliath':
… the myth of the emerging State of Israel as David facing the Arab Goliath in the 1948 war … Despite seemingly overwhelming demographic advantages, the Arab states were not prepared for conflict. Jewish forces consistently outnumbered Arab armies – often by a factor of two-to-one – enjoyed better access to arms, maintained shorter supply-lines, and were far more experienced than their opponents having fought against and alongside British forces under the Mandate and during World War II, respectively.
The Arab states, in contrast, were fighting their first-ever war; the Palestinians, for their part, were almost totally disorganized. Thus, from a purely military standpoint, a Jewish/Israeli victory was all-but-assured.
Source: Benny Morris, cited in 'Review: 1948: A History of the First Arab-Israeli War' by Paul Chamberlain.
Despite the huge population disparity, Israeli mobilization was much higher and
None of the Arab states were willing to commit their resources fully to the destruction of Israel, and ultimately no more than 150,000 soldiers, the majority of whom were Israeli, entered the war despite the involvement of five armies.
Had Arab states committed more forces against Israel's long border, they may well have stretched Israeli resources beyond breaking point. Even so, the Israelis were hard pushed during the first phase, yet managed to hold their own:
All in all, the combined and simultaneous Arab invasion turned out to be less well-coordinated, less determined, and less effective than Israel’s leaders had feared. Success in withstanding the Arab invasion greatly enhanced Israel’s self-confidence.
British-Israeli historian Avi Shlaim observes that "the first truce was a turning-point in the history of the war":
Though they [the Israelis] had succeeded in halting the Arab invasion, their fighting forces were stretched to the limit and badly needed a respite to rest, reorganize, and train new recruits. On the Israeli side, the four weeks’ truce was also used to bring in large shipments of arms from abroad in contravention of the UN embargo – tanks, armoured cars, artillery, and aircraft. On the Arab side, the truce was largely wasted. No serious preparations were made by any of the Arab countries to reorganize and re-equip their armies so that they would be better placed in the event of hostilities being resumed. The UN arms embargo applied in theory to all the combatants but in practice it hurt the Arabs and helped Israel because the Western powers observed it whereas the Soviet bloc did not. … It witnessed a decisive shift in the balance of forces in favour of Israel.
The arms that Israel obtained (but which the Arab states largely couldn't) came from the Eastern Bloc, principally Czechoslovakia with whom the Israelis had signed contracts despite the arms embargo. This violated the UN arms embargo (actively supported by the US and the UK – see this History SE post), imposed in an attempt to de-escalate conflict in the Middle East. This violation was raised in the UK parliament where the use of Skoda rifles and Yak fighters is mentioned (see this Hansard entry for more on the UK government's response to the violation of the embargo). Thus, although the Arab states could have made better use of the truce in terms of more coordinated planning and preparation, Shlaim's use of the word 'wasted' is perhaps a little harsh in view of the greater difficulty the Arab states had in obtaining extra supplies. As Amitzur Ilan states in The Origin of the Arab-Israeli Arms Race: Arms, Embargo, Military Power and Decision in the 1948 Palestine War,
… although the Arab armies started the war better equipped than the Israeli army, their organization was from the start badly lacking and their stores of ammunition and spare parts were nearly empty.
Thereafter, due to embargo restrictions, their former sources of supply and military know-how were cut off … In contrast, the Israeli forces, which were also badly hit by the embargo, nevertheless managed to import a considerable number of weapons and supplies and to produce some at home as well, and to attain considerable foreign military expertise.
Although the Soviet Union itself did not respond to Israeli requests for arms, it did not stop states in the Eastern bloc from assisting Israel. Principally, this meant Czechoslovakia (with the co-operation of Hungary and Yugoslavia in transporting the weapons). Arms were supplied to both sides, but it was the Israelis who got the bulk of them. The motive for breaking the embargo appears to have been primarily Czech financial need:
...it was a policy motivated by strong mutual interest which was that
the Yishuv desperately needed arms which the Czechs were keen to
export and the Czechs desperately needed dollars which the Yishuv
could raise in the USA. True, the first contact between the Yishuv and
the Czech Government began when there were a number of pro-Zionists in
that government. But that same government also offered military
assistance to Syria and Egypt and the military assistance to Israel
continued long after the pro-Zionists were no longer in government.
The fact that in 1948 Israel received 85% of the Czech foreign
military aid and the Arabs only the remainder, does not indicate any
degree of preference but who was the better customer. A 'reliable source' in the Czech Foreign Office, which used to pass information to the British
Military Attache in Prague, said categorically that the Czech interest
in exporting arms 'is just Dollars' and that Moscow understood the Prague position.
Source: Amitzur Ilan
Shlaim also points to the lack of unity among the Arab states, citing several examples at various stages of the war in his article Israel and the Arab Coalition in 1948. For example, when Israel attacked Egypt on October 15, Transjordan remained neutral while the Arab legion failed to assist the Egyptian forces trapped in Faluja (Gaza). Then, when the Israelis attacked for the second time at the end of October,
Conflict between the Arab states and lack of coordination between their armies in Palestine gave Israel the freedom to choose the time and place of the second offensive. Egypt appealed to her Arab allies for help but its appeals fell on deaf ears. Lebanon, Saudi Arabia, and the Yemen all promised assistance but failed to honour their promises. The Iraqis shelled a few Israeli villages near their front line as a token of solidarity with their embattled ally. Without exception the Arab states were either afraid to intervene or did not wish to intervene.
The Egyptians should not have been surprised at this lack of support: back in May, an Arab Legion, despite being stationed nearby, failed to assist the Egyptian advance towards Jerusalem. This view on Arab disunity is supported by the Syrian historian Sami Moubayed:
The rivalries were a major problem because they resulted in poor command, lack of transparency, and ultimately, failure, …
and by Professor Arthur Goldschmidt Jr who argues that the rivalries changed the course of the war:
Notably the rivalry between the Jordanians, with their British-officered Arab Legion and King Abdullah’s ambitions for a Greater Syria, and the Egyptians, with King Farouk’s ambition to lead the Arab World, backed to some degree by the League of Arab States and by the former mufti of Jerusalem …
(All emphasis is mine)