Answer
The heartland of the Byzantine Empire in Anatolia was lost due to multiple factors during the reign of Andronikos II Palaiologos. The Seljuk Turks took advantage of the empire's internal conflicts and Mongol invasions to expand their territories, capturing key cities like Dorylaeum and Nicaea. Internal strife, economic decline, and the emperor's ineffective military strategies further contributed to the loss of control in the region.