Hebrews 10:39

But we are not of a disposition to draw back so as to perish, but we have faith that leads to the saving of the soul.

Luke 17:31-32

The man who is on the roof of his house that day, and his goods in the house, must not come down to carry them out; and the man in the field, too, must not turn back.

Acts 15:38

Paul, however, did not consider such a man fit to take along with them, the man who deserted them in Pamphylia and did not go on with them to the work.

2 Timothy 4:10

for Demas has forsaken me because he loved the present world, and has gone to Thessalonica; Crescens has gone to Galatia, Titus to Dalmatia;

Treasury of Scripture Knowledge

Summary

Because they turned back from him, and would not consider any of his ways:

Bible References

Turned

Luke 17:31
The man who is on the roof of his house that day, and his goods in the house, must not come down to carry them out; and the man in the field, too, must not turn back.
Acts 15:38
Paul, however, did not consider such a man fit to take along with them, the man who deserted them in Pamphylia and did not go on with them to the work.
2 Timothy 4:10
for Demas has forsaken me because he loved the present world, and has gone to Thessalonica; Crescens has gone to Galatia, Titus to Dalmatia;
Hebrews 10:39
But we are not of a disposition to draw back so as to perish, but we have faith that leads to the saving of the soul.