Because Jews and Christians (classicly) dont use the same calendar. Passover is on the 15 of Nissan the first month of the Hebrew calender and easter is calculated to be on the first Sunday after the first full moon after the march equinox (march 21 first). To go a bit deeper the Hebrew calender has in most years just 354 days instead of the 365 days of the gregorian calendar, because they follow the moon cycle. But every two or three years an extra month is added to get back into rhythm with the sun year. So both dates are calculated based on the moon but since the length of the years differe Easter and Passover also differ.
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