because Jesus's death was linked to the passover which is based on Jewish calendar which was lunisolar therefore date of passover changes with the moon and so does Easter because the different way some interpret the full moon Easter can be different in the Eastern Orthodox church
the Jewish months are lunar so some years have 13 months instead of 12 then there are 2 months called Adar, in fact this year is one of them ( there are Adar I and Adar II, 7 times in 19 years)