why was it a heartache for Paul that those in Asia turned away from him?
It was a heartache because Paul had suffered beatings, imprisonments, and shipwrecks to bring the Gospel to people who had never heard, saw them established in small home assemblies, and then all in Asia turned away from him. Their departure was painful repayment for his sacrificial labor.
Les explains that this must have been a heartache for the man, Paul, after all he had endured—the sufferings, beatings, imprisonments, and shipwrecks—simply to take the Gospel to people who had never heard. He saw them established in it and saw churches established, though they were not large. They were small assemblies meeting in homes, unlike the large church buildings people often think of today. Yet in II Timothy chapter 1, verse 15, he says that all they who are in Asia had turned away from him. Les notes that Asia there is primarily what today is Turkey, with Galatia in the area between the broader marks. The pain came from their turning away after such costly labor and establishment.
This thou knowest, that all they which are in Asia be turned away from me; of whom are Phygellus and Hermogenes.