I lately heard from some people, they stopped giving PESEL number to foreigners except for people who have permanent residence card, in the past when you register yourself in Urzad Miasta you were able to tell them you would like to have a PESEL number and after a few weeks you could go back and ask them for PESEL confirmation..
It's the case for non-EU citizens for certain. Not sure about EU citizens.
Anyway, the reason is that your ID card number functions as your PESEL on governmental databases.