It's not "coincidence" at all. The 14th day of Nisan is the 14th day after the new moon, so it's at or very close to a full moon. Easter is the Sunday after the first full moon after the vernal equinox, March 22. It's basically the same dating system, adjusted (a) to fall on a Sunday; and (b) adjusted to be computable from the Greco-Roman solar calendar instead of from the Hebrew lunar calendar.
And, BTW, if Easter and Passover are unrelated, why is Easter called "Passover" in Greek, Latin, and every language derived from Latin?
Please explain why the Nicean canons intentionally forbid the biblical Passover when deriving the date for the pagan holiday of Easter. Please explain why there was even such a thing as the Quartodecimian Controversy in the 2nd Century. The fact is, considerable church history was spent trying to stamp out the celebration of the biblical Passover, and disconnecting from Passover on the common calendar was mandated in the Nicean decrees. It is not as simple as your “it’s the full moon” excuse.