Ataturk, the first President of Turkey, established his resistance movement against the Ottoman Empire in Ankara. After the fall of the Ottoman Empire, when the Turkish Republic was founded, Ataturk established his former headquarters as the capital.

Part of the reason is that Ataturk's resistance movement was initially based in Anakara. There were also other factors, one of which was to create a distinction between the archaic Ottoman Empire and the much more modern Republic of Turkey. Ankara was also far more defensible against outside attack.

Everything else here is correct, but there's also some geographic considerations--Istanbul being on the Bosporus makes for a great geographic position for trading, but is also tough to defend (you can sail right up and get close from both sides). Ataturk moved the capital inland to the hillier Ankara with the thought in mind that it would be a more defensible capital.