Apricot is much less popular in Poland, but I'll definitely try it!
I remember while travelling via Czech Republic, apricot stands were really common in southern regions. It's just a wild guess, but these might be slight climate differences I wouldn't pay attention to otherwise so close to my country. It was a region south from Brno.
I think you are actually right with your theory. Apricots are definitely much more common in the (sunny) southern parts of the country, especially in South Moravia (which is where Brno is located) than in the northern regions so I can imagine that up there in Poland, apricots are grown only very rarely... But you should really try these apricot dumplings, they are fantastic :)