why aren't there any Polish-based singers to make it big internationally?
hmm ...what do you mean internationally? Define it? Think it over will you?
For a couple of decades the international market has been dominated by the American pop and it entertainment industry, if you wanted to make it big you had to make it big in the USA. That trend is changing, maybe not so much in Europe but elsewhere.
I find you question to belong the same category (weird and useless) as why there is no Polish actors in the Hollywood. Poland ain't America dude, duh!
A More serious answer so called language barrier.
Inna and Alexandra Stan
By the way. Never heard about them. You should reconsider your approach as you basic assumptions are false.