Question

Why had Islam not spread into Asia Minor (western and central Turkey) by 850? When and why did Islam eventually spread into this part of Asia?

Answer 1

The Byzantines proved to be stronger than previous Muslim enemies. The Abbasids came to power among the muslim empire in about 750. The Abbasid empire stretched from Arabia to Spain. Conflict along the Anatolian frontier was not the only area of focus.
Things stabilized a bit between Byzantines and Arabs. They exchanged ambassadors and had truces at times.
By the 11the century (1060) the Abbasids had been replaced by Turks and Mongols. The Seljuk Turks moved into Asia Minor at that point. The Byzantine Christian empire kept shirking until the moslem turks finally took Constantinople in 1453. The Turks were known as Ottomans by then.