Hi
Flappie, long time no see!
There could be many reasons such as bigger pay packet. Or maybe as simple as wanting to change the view - ie going to different places. Bigger might not necessarily be better, but it is the natural order of progression, for many folks, to fly bigger and bigger aircraft.
It could even be as simple as boredom in the current job that motivates a move. Or maybe the bases on offer with the new aircraft job are better places to live.