First, Boleslaw would have had to unite Poland - and AFAIK, his chances to actually achieve that were rather poor. Second, Lithuania was not Christian at the time. Mendaugas was indeed baptized in 1251 but it didn't last long - he returned to pagan faith in 1261. Third, at the time Lithuania was not so big and already in conflict with the Teutonic Order and Livonian Order (and Polish duchies at the time had quite good relations with them) - too much trouble for too little gain,