One of the contingent of this army was serbian, and they were very, very motivated to take on Austria-Hungary, liberate their homeland from the harsh austrian occupation, and gain control over territory they were promised (roughly, Herzegovina, Krajina, Voivodina). Don't forget that when Bulgaria surrendered (at the end of september) Serbia was still occupied by Austria-Hungary.

The other motives were that the Ottoman was seen as a much less threatening foe than Austria-Hungary (for good reasons):

• Ottoman armies were supplied in ammunition through the berlin/istanbul/bagdad railway. With Bulgaria and (soon) Serbia under Entente control, the only way this could be accessed for the Alliance was through Ukraine or Romania and the Black Sea which was difficult, not to say impossible (bad transportation, Entente on Romania's doorstep, complicated situation in Ukraine...).

• There was an ongoing arabian revolt. In the eye of the Entente, the ottoman empire was on its last breath, so why bother too much?

• It would have implied to crack Edirne/Adrinapole. Not the toughest nut, but still, it would have required a siege and probably the concentration of Entente troops. Remember that it wasn't that big of an army, so it would probably have prevented any other major operation until the fall of the city.

• Even if the Entente took down the sick man of Europe, there was not much to gain: the Alliance didn't depend on critical supply from the ottomans and it was a negligeable military force at this time of the war.

• This one is more in retrospect, but there may have been the belief that, anyway, Greece could handle the weakened ottoman alone (which would be proved wrong)

• With Russia out of the war, the main reason to take Constantinople (supplying Russia) wasn't there anymore.

On the other hand, pressuring towards Austria and Hungary was much more interesting:

• Cutting through the agricultural heart of the austrohungarian empire, and thus exacerbing the food problem of the Alliance.

• Supporting the italians that were preparing a big offensive against Austria-Hungary (the soon to be Vittorio Veneto), by forcing Austria-Hungary to transfer more troops to the Balkan front. Remember that at this time, Austria-Hungary was still quite deep in Italy.

• This one is more "looking back" but still, hastening the decomposition of Austria-Hungary and threatening directly Germany on a new front, forcing more german troops to move there and easing the advance of the Entente troop in the West with the "hudnred day offensive".