Because in reality its the same as British riders going abroad and in the main struggling. Okay, you have your Bewleys, Lamberts and Woffys who succeed but how many others hold down regular spots in the top leagues over seas?
If you watch racing from Swedish and Danish tracks, you will realize they aren't actually that similar to UK tracks. And Polish tracks definitely aren't.
It's what makes it even more impressive when you see a foreign rider come to the UK and hit the ground running straight away.