Easter is a Germanic word for "resurrection," which was in common use in the era that this translation of the Bible was brought into completion. That is why the translators of the Authorized Version used it, most likely viewing the Passover as a Jewish observance whereas the resurrection of Christ was celebrated by Christians.
The Greek Pascha is a transliteration of the Hebrew Pesach. Interestingly, this term is translated as "Passover" every other instance that it occurs in the King James Version. Easter is a later tradition taken up by Christians. Consequently, "Passover" is a better choice of words than is "Easter" in Acts 12:4.