Was Poland a duchy for a period of time? All of these rulers seem to be Dukes, not Kings, especially since there was also a Wladyslaw III who was a king.
During that period, a Duke needed a papal permission to crown himself a King. That's why Bolesław the Brave, Mieszko II, Bolesław the Bold were Kings, while others just Dukes. The same applied to Moravia, but as far as I know, only one Duke managed to upgrade his title. Starting with Władysław the Elbow-High all subsequent rulers were Kings. After first personal, then real union with Lithuania Polish rulers were both Kings and Grand Dukes of Lithuania, which never became Kingdom on its own. That's why Poland was being frequently refereed to as simply "the crown".
Interestingly Bolesław the Brave only got the crown in the last year of his life. During the feudal partition following Bolesław the Wrymouth's death, Poland became a loose union of semi-sovereign duchies with the Duke of Kraków being a "High Duke" or "Duke-Senior". It was a total mess, people.