It's largely because of the Quartodeciman controversy in the 2nd through 4th century. Prior to this there was some diversity of when to celebrate Easter, with Christians in Asia (Minor, that is), celebrating Passover on the 14th Nisan, aligning with Passover according to the Jewish calendar. Christians in other districts, including Rome, should be celebrated on a Sunday, rather than being completely movable. A number of synods dealt with the issue, proclaiming various decrees. It cam eto be settled that it should be the first Sunday following the Paschal full moon after teh equinox. Western churches do this according to the ecclesial date of 21st March, and follow the Gregorian calendar. Eastern churches (Orthodox, and some others), determine it according to the Gregorian calendar.

The council of Nicaea in 325 did fix the date and practice, basing it primarily on the Julian solar calendar, rather than continuing to follow Jewish lunar calendrical practices.

tl, dr: dominant Christian practice was to celebrate it on a Sunday, after Passover, not 3 days after Passover which kept moving which day of the week it was.