Because it's based on the lunar calander and the moon doesn't wax and wane in accordance with the Julian calender.
This is because the Bible mentions the last supper taking place a certain number of days after the passover, and using this, it's possible to work out the date.
I believe that in the Orthodox Church, the date for Easter is fixed. In fact, I think this was the cause of the major split between the Eastern and Western Churches. In the Western Church, the link between Easter and Passover is maintained, reminding us that Jesus was Jewish and the the Last Supper happened at Passover. It reminds us too that God saved the firstborn of the Jews in Egypt during the tenth plague but sacrificed his own son to redeem mankind.