Players move overseas for two reasons

Some consideration should be given to the professional player. He has a short span career and has to make the most of it.

If you are the 2nd or 3rd best full back, winger, centre, fly half, scrum half, loose forward, hooker or prop in the world, and you just happen to be a native of the same country as the best or second best player, it ain't rocket science to see that you would advance further in a country that doesn't have a player as good as you in your position.

If you move, you advance further.

Players tend move countries for two reasons, the one above and for a few extra pay days, at the club level, in the twilight of their career's. These twilight moves tend to be accepted easier, look at the current migrations to Japan.