As the independence war started in Anatolia, Istanbul which was Ottoman Empire's capital was occupied by England and Ankara was far away from the invading forces, not only that but also it was close enough to supply the troops without much hassle. So because of the aforementioned reasons, Ankara was selected as the new capital of Turkish Republic.

Other posters have mentioned the strategic reasons for doing so, but there were also nationalistic/ideological ones.

In short Turkish nationalists saw Istanbul as a decadent, multicultural Imperial city filled with symbols of Islam and the Sultan's regime but lacking in true Turkish character. Ankara was seen as a truly Turkish place where the new nationalist, secular regime could have a fresh start.

The Turkish nationalist movement that Ataturk represented did not see the Ottoman empire as a truly Turkish polity, rather they saw it as a backwards, repressive, multicultural empire run by a monarch with no true nationalistic loyalty to any people.

Many Ottoman Sultans had non-Turkish mothers, lovers and advisers and even though contemporary Europeans generally saw the Ottoman Empire as "Turkish" many contemporary Turks, especially those in the provinces, did not think the court in Istanbul reflected their interests or culture.

Connected to this is the fact that Istanbul has historically not been a particularly "Turkish" city. We all know it was founded by the Greeks, but it retained a significant Greek presence until the 20th century. It was also home to the myriad of other ethnicities who were ruled by the Ottomans.

When Ottoman empire disintegrated and the Greeks and Turks fought their war, Greek leadership tried to annex Istanbul (along with most of coastal Anatolia) and had the support of many powerful western governments. Had history been a little different, it's possible Istanbul would be Constantinople again.

While Greeks did have significant historical ties to coastal Anatolia, they had much less claim in the landlocked interior of Turkey which had been majority Turkish since well before the fall of Constantinople.

These days many Turks look back to the Ottoman times with pride, but when Ataturk took control, Turkish intellectuals were ashamed of what they saw as the backwardness of the Ottoman Empire and wanted to disassociate their new modern, secular nation from it. Ankara is deep in the Turkish speaking heartland and is also roughly in the middle of Modern Turkey, so it was a perfect place to start.