They seem to be coming due to perception of an easier, more direct route into the NBA. And it helps that they'll play in front of much bigger crowds than they did in Europe/Spain/Israel.

Still, if their minutes turn out to be less than they assumed and the hotels in Stillwater, Ames and Lubbock look just like the hotels back home, and they're miserable in American Heritage... the grass may look greener elsewhere to someone that has already played for a lot of teams.