I would guess that money, frequent travel and/or childcare difficulties would all be plausible reasons why a parent might feel that their child is better off with the other parent. Of course, these are all very circumstantial reasons that will vary from parent to parent.

One parent having custody also doesn't mean that the other parent will never see their child. There are different types of custody. I know parents who have primary custody, and the other parent has the child for summers/holidays.