As the 1st World War ended and the Ottoman Empire signed the Mondros Ceasefire Agreement which has heavy conditions.
At that time Mustafa Kemal ,who was in İstanbul,(Atatürk) was not satisfied with the occupation of the country.So he went to Samsun on 19 May 1919 and started the Turkish Independence War with his comrade-in-arms.
According to the circumstances it was quite possible to occupy the western part of the country. ( Istanbul and Izmir were occupied later.)This made the inner Anatolia Region a more wiseful choice to select it as the “ Headquarter”
Thus,the Independence War was ruled from Ankara and the first parliament (Grand National Assembly of Turkey) was established in Ankara during the time of the war.( It was active even during the war )
Also,Ankara had telegraph lines between east and west.This facilitated communication during the war time.
As I mentioned earlier, Ankara is in the inner Anatolia region of Turkey.So Ankara was more difficult to be occupied than Istanbul.
And the last reason :
Even in the war years,Mustafa Kemal(ATATURK) was aiming to establish a new Turkish State.The Ottoman Empire had no place in history anymore.The new state should not be a continuation of the Ottoman Empire.A modern and exemplary state was needed.If Istanbul was chosen to be the capital of the country,the Turkish State would seem like a continuation of the Ottoman Empire.
Hence,Ankara where the Turkish Indepence War was ruled and the parliament stays selected as the capital city.
For last, here is a map of the Turkish Republic. Here you can see where Ankara and Istanbul is.
Hazel Fern