Turkey's predecessor state, the Ottoman Empire, had its capital in Istanbul. Are there any reasons for the capital's move when it became Turkey?
During the early stages of the Turkish War of Independence, Kemal commanded two army corps: One in Ankara and one in Erzurum. At first Erzurum was Kemal's base of operations, but on 27 December, 1919 he moved his Representative Committee to Ankara to be closer to Istanbul.
Ankara also had a railway and being roughly at the center of Turkey made more sense as a capital logistically. Eskişehir, which also had a railway and was close to Istanbul, was either very near the border or occupied by the Triple Entente and its proxies during various stages of the war.
In April 1920, Kemal established a parliament in Ankara, officially making the city his capital. After the nationalist victory in 1923 and the overthrow of the Sultanate, moving the capital back to Istanbul didn't make much sense. Kemal enjoyed popular support throughout Turkey, but it was in Ankara - his war capital - where his support was the strongest. In contrast, there were still remnants of Ottoman elements in Istanbul, who may have tried to sabotage the new regime if given the chance.
Furthermore, the city's position in the middle of the Rumeli (west) and Anadolu (east) territories allowed for more efficient central control of the young Republic. Also, having the capital as far away from the Republic's enemies (Greece, mainly) as possible was certainly a bonus.
Lastly, keeping the capital in Ankara certainly had symbolic value. Kemal had dissolved a six hundred old empire and was attempting to revamp the entirety of Turkish culture. Moving back to the old capital wouldn't have helped that cause, while keeping the new capital sent a strong signal that this was a new era for Turkey.
The reasons probably relate to World War I. There were three cities in northern Turkey of "capital" importance: Istanbul in the northwest, Ankara in the north center and Erzurum in the northeast.
Erzurum was captured by "enemies" (Russia) during World War I, and Istanbul nearly so, by the British attack on Gallipoli. Both of these cities were also threatened during Turkey's War of Independence. Ankara was the safest, most secure city of the three, and was therefore the logical choice.
Because that's what Ataturk wanted when he created the new modern Turkey. A simple Google search would have clarified that for you (not trying to offend just saying). Ankara is also in a strategic position should another country try to invade it since it's literally in the centre of Turkey! Mind you would you rather have your capital exposed like Istanbul to the sea to Russians and everyone else or Ankara that takes more or less the same long time to reach between all turkey's sides?
When the two cities Erzurum and Istanbul were invaded by the western nations USSR and UK respectively, there was only one safest place for the Turks to live in, Ankara. This was during WW1. Therefore, Ankara is still the capital of Turkey, even though Istanbul is the largest city in Turkey.