Why do Christians not celebrate Passover?

1Cor:5:7: Purge out therefore the old leaven, that ye may be a new lump, as ye are unleavened. For even Christ our passoveris sacrificed for us:

Luke:22:14-20: And when the hour was come, he sat down, and the twelve apostles with him.

15: And he said unto them, With desire I have desired to eat this passover with you before I suffer:

16: For I say unto you, I will not any more eat thereof, until it be fulfilled in the kingdom of God.

17: And he took the cup, and gave thanks, and said, Take this, and divide it among yourselves:

18: For I say unto you, I will not drink of the fruit of the vine, until the kingdom of God shall come.

19: And he took bread, and gave thanks, and brake it, and gave unto them, saying, This is my body which is given for you:this do in remembrance of me.

20: Likewise also the cup after supper, saying, This cup is the new testament in my blood, which is shed for you.