Why was Belgium created as opposed to grouping the Fracophone Walloons and the Dutch-Speaking Flemish with their respective linguistically similar neighbor countries; France and the Netherlands?
First, Belgium wasn't created by uniting the Walloons with the Flemish, but by secession from the Netherlands. This event is known as the Belgian Revolution.
According to the linked Wikipedia article, one of the reasons for the revolution was that many future Belgians, even Flemish, "regarded King William I's rule as despotic".
Moreover, Belgians are Catholic, while the Dutch are protestant. The Belgian Revolution happened in 1830, when nationalism wasn't very popular yet, so linguistic similarity didn't play as big a role as a common religion.
This explains why both the Walloons and the Flemish wanted to secede from the Netherlands. Why not join France? I don't have a good answer, but again, since there was little nationalism (or nationalism wasn't linguistic), there was little reason to do so.
In the middle ages, Belgium and the Netherlands belonged to an area called the "Low Countries". In 1384 this area came under the dominion of the Dukes of Burgundy, starting what is known as the "Burgundian Netherlands". These domions were merged into the Spanish crown as a result of the political marriage of the Holy Roman Emperor Maximilian I to Mary of Burgundy in 1477. Their son Philip "the Handsome" became King of Castile due to his own marriage to Juana "the Mad" of Castile. He inherited the territory and passed it on to his son with Juana who became simultaneously the Holy Roman Emperor Karl V and the King of Spain (formally King of Castile, Leon and Aragon) Carlos I. This personal union ended with Carlos' son becoming king of Spain Philip II and Carlos' uncle (son of Philip I) becoming Holy Roman Emperor Ferdinand I. The settlement was that the Low Countries remain with Spain (the "Spanish Netherlands")
In 1568 the Dutch Revolt erupted with seven north provinces of the Spanish Netherlands splitting from Spain and becoming the Republic of the Seven United Netherlands (the Dutch Republic). From this point the histories of Belgium and Netherlands diverge. The roots, however, are in earlier differences, in particular the dominance of Catholic Christianity in the south and Calvinism in the north. In 1648 Spain recognized the Dutch independence in the Peace of Westphalia which ended the Thirty Years War. The south part remained Spanish, known as the "Southern Netherlands".
In the aftermath of the War of the Spanish Sucession, in 1713, the Southern Netherlands passed to Austria in compensation for the Austrian claim on the contested Spanish crown. During the War of the First Coalition the First French Republic conquered the region from Austria in 1794. The conquest was recognized by Austria in the Treaty of Campo Formio ending the war in 1797. After the French defeat in 1814, the Congress of Viena formed a new Netherlands monarchy headed by William I, previously Prince of Orange (the Princes of Orange were semi-official monarchs of the Dutch Republic). This monarchy covered the entire Low Countries region, thus unifying south and north again. However, the union didn't last long. In 1830 the Belgian Revolution erupted for numerous reasons including under-representation of the south in the States-General (the north and the south had an equal number of representatives even though the south had a somewhat larger population). The south thus became an independant Kingdom of Belgium with the backing of France (which just had its own revolution: the July Revolution which replaced the House of Bourbon with House of Orleans). Leopold of the German house of Saxe-Coburg-Gotha was crowned as king in 1831.
The creation of Belgium STARTED when the (Protestant) Netherlands seceded from the (Catholic_ Spanish Netherlands in the 16th century, reducing it to modern "Belgium," under Hapsburg rule.
After the Napoleonic wars, "Belgium" was briefly reunited with the "Netherlands" in a forced "merger" as part of the peace settlement. It was unhappy under Dutch rule, rebelled in 1830, and became an independent country in 1839.
The Kingdom of The Netherlands was formed in 1813 to form a buffer against France. The Belgians weren't asked anything. King Willy 1 (William of Orange) was simply a good negotiator. The allies didn't want to do it all over again, so uniting (now) The Netherlands with (now) Belgium seemed a good idea.
There are many reasons why it didn't work out:
- William 1 didn't endear himself with the Belgians, even at his coronation. He was protestant (Belgians were Catholic) and stingy as hell. It was customary to throw golden coins during the coronation ceremony, but William opted for copper coins. His nickname became 'the copper king'.
- Difference in economics. The Netherlands was mainly agricultural and trade orientated, in Belgium the industrial revolution was already beginning. The Belgians had the idea that they (Catholics) paid for the (Protestant) kingdom.
- The French very much wanted Belgium to secede. The Allies liked a strong Netherlands on the border of France, but France of course had different ideas. As soon as the revolution broke out, the French army moved in to assist the poor oppressed Belgians in their quest for independence.
In fact, it wasn't so much the brave Belgian resistance or army who got their country independent, but far more the large French army supporting them.
In Belgium was (and is) the language barrier between Dutch and French. Half the population speaks Dutch, and the other half French. During the Belgian revolt, the French speaking part took control of the country. Almost the entire nobility today is of Walloon origin. Many Belgians say there is only one Belgian: the king himself. Everybody else is either Walloon or Flemish.
Of course the Dutch suggested that the Dutch speaking population should become Dutch. But the allies didn't want that. They didn't want French to grow by adding Wallonia. Prussian wasn't keen to see The Netherlands grow by adding Flanders to it. The Belgians themselves were pretty much fed up by being ruled by the Dutch, so that was about the last thing they wanted anyway.
It took a long time before negotiations were finished. King William almost bankrupted the country by keeping the army fully mobilized for 9 years. Only when the country was on the brink of declaring bankruptcy he agreed with the partition. Luxembourg became a personal property of king William until the death of king William III.
Sources (mainly in Dutch):
Belgium seceded from the Netherlands under treaty in 1830. This was intended at the time to establish a permanent protestant majority in the northern part of the Netherlands, by splitting the existing Roman Catholic majority, who, for their part, were unhappy with the ruling dynasty's attempts to create an establishment protestant church. It probably had that effect at time. However, the small protestant majority created in the north probably only lasted for a relatively short while, as subsequently there has almost always been a small Roman Catholic majority in the (remaining) Netherlands, at least amongst those remaining who claim to be Christian. - There are echoes in the UK's creation, for similar reasons, of Northern Ireland in 1921.