I Hate to be boring and add a serious answer to an otherwise interesting thread, but it's because it's linked to the Jewish festival of Passover. And Passover is linked to the lunar calendar.
Basically Jesus' last week alive coincided with passover, and so Easter still coincides with passwover (with some complications IIRC).
While some bits of Easter may be pagan (eggs and bunnies etc) the date isn't, it's Jewish. Sorry!