1 Corinthians 5:7 Purge out therefore the old leaven, that ye may be a new lump, as ye are unleavened. For even Christ our passover is sacrificed for us:
8 Therefore let us keep the feast, not with old leaven, neither with the leaven of malice and wickedness; but with the unleavened bread of sincerity and truth.
Why? Christ took the place of a real lamb for a real feast (Passover).
Paul would not use leavened or unleavened bread regarding a literal feast knowing that it was part of the law done away in Christ.
Why? The real feast used unleavened bread.
Christ's sacrifice was of a piece with the Passover Seder. Why did Christ institute the new covenant within the boundaries of the old (the passover supper, that is)?
You are correct that Easter (and to a similar extent Christmas) have become flooded (and in some cases buried) with pagan ritual signs. But that does not change the real celebration of His death burial and resurrection. Christmas is harder since it does not even correspond to the actual birth day of Christ, but Easter is forever tied to the Passover.