They always explain things in Star Trek, but this is one of the few things I have found no answers for!! LOL!!
Becuz Crusher had finally squared away her feelings for Jean Luc and besides...Pulaski STUNK as the dr on STNG
Dr. Crusher was the head of Starfleet medical from 2365-2366 and the ship obviously needed a new doctor so along came Pulaski.
Beverly Crusher has to serve as the interium head of StarFleet Medical for a year, until they appointed someone else. Then she came back to the Enterprise. That's why Dr. Pulaski was assigned to the Enterprise.
Unlike the responders above, I'm sure you're looking for the explanation as for why Crusher came back (she showed up in Season 3, by the way; Season 2 was all-Pulaski) and Pulaski left, not why Crusher left. For that question, there was never an in-show explanation.
She covered for Crusher while Crusher was took command of Starfleet medical on earth for a small time.
