Because it's theologically important that Easter be celebrated the Sunday after Passover, and Passover is reckoned by the Jewish lunar calendar which is different to our ordinary solar calendar.
If the date of Passover and therefore Easter is pegged to the Jewish calendar, why are there different dates for Easter in Eastern Orthodoxy vs. Western Christianity?
I don't believe the date is different: its just that the Julian calendar is out-of-sync with the Gregorian calendar.
But why should that matter if the day is pegged to a calendar which is neither Julian nor Gregorian?
I believe that the dates for Easter were calculated and set centuries in advance. The Gregorian calendar then recalculated them but the Julian calendar using countries did not and once they started using the Gregorian calendar the Orthodox church still insisted on keeping the same dates.
