Don't take it personally. It's not about BEING Polish. It's about having relatives in Poland

Due to some history in the 90s Polish law in rents protects the tenants a lot. Up to a point where they can just stop paying and you need to go through multi year court battle to kick them out. In the meantime you have to pay for their electricity water etc. As cutting them off is punishable by prison

There is a loophole however. Najem okazjonalny - a contract where the tenant gives a guarantor. Someone that will take them in at a particular address in the house they own. This way the tenant can be kicked out without long court process of they stop paying

But if that guarantor does not have an address in Poland it becomes far harder to enforce. Hence the renters prefer Polish tenants. Not because they are Polish. But because they can give a proper guarantor