They do not celebrate it because they have been taught, falsely, that it is a Jewish Holiday and does not apply to them as gentiles. However, it is plain that Paul taught the Corinthian Chruch, a group of gentile converts, to keep the Passover and the Days of Unleavened Bread.
1 Corinthians 5:6-8 KJV (6) Your glorying is not good. Know ye not that a little leaven leaveneth the whole lump? (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.
That is purely a Festival of Unleavened Bread sermon! (Leviticus 23:6)
Further, he tells them to celebrate the Passover (Leviticus 23:5) with the new symbols of bread and wine as instituted by Christ on the night He was betrayed:
1 Corinthians 11:17-29 KJV (17) Now in this that I declare unto you I praise you not, that ye come together not for the better, but for the worse. (18) For first of all, when ye come together in the church, I hear that there be divisions among you; and I partly believe it. (19) For there must be also heresies among you, that they which are approved may be made manifest among you. (20) When ye come together therefore into one place, this is not to eat the Lord's supper. (21) For in eating every one taketh before other his own supper: and one is hungry, and another is drunken. (22) What? have ye not houses to eat and to drink in? or despise ye the church of God, and shame them that have not? What shall I say to you? shall I praise you in this? I praise you not. (23) For I have received of the Lord that which also I delivered unto you, That the Lord Jesus the same night in which he was betrayed took bread: (24) And when he had given thanks, he brake it, and said, Take, eat: this is my body, which is broken for you: this do in remembrance of me. (25) After the same manner also he took the cup, when he had supped, saying, This cup is the new testament in my blood: this do ye, as oft as ye drink it, in remembrance of me. (26) For as often as ye eat this bread, and drink this cup, ye do shew the Lord's death till he come. (27) Wherefore whosoever shall eat this bread, and drink this cup of the Lord, unworthily, shall be guilty of the body and blood of the Lord. (28) But let a man examine himself, and so let him eat of that bread, and drink of that cup. (29) For he that eateth and drinketh unworthily, eateth and drinketh damnation to himself, not discerning the Lord's body.
The above also contains a warning about the attitude we should have in taking that bread and wine. This is NOT communion, this is a once yearly memorial of the Messiah's death.
Paul even used a High Day (Atonement Leviticus 23:27) to show the time of year that he was sailing on one of his Missionary journeys:
Acts 27:8-9 KJV And, hardly passing it, came unto a place which is called The fair havens; nigh whereunto was the city of Lasea. (9) Now when much time was spent, and when sailing was now dangerous, because the fast was now already past, Paul admonished them,
It is plain from Colossians 2:16 that Paul was teaching his gentile converts to observe the Sabbath, Festivals, dietary laws, and new moons. the real proof of this is the inclusion of "new moons". there would have been no need to teach about "new moons" except to make sure the congregations he established were able to determine the correct dates of the Festivals for themselves. Passover is 14 days after a new moon. Trumpets (Leviticus 23:24) starts on the new moon.
Colossians 2:16 ESV Therefore let no one pass judgment on you in questions of food and drink, or with regard to a festival or a new moon or a Sabbath.
Why was this written? It is because Paul was teaching pagan, gentile converts to observe Torah and to persevere in the face of persecution from the pagan communities in which they lived. It is not written to absolve folks from doing these things, Paul's letters are for encouragement to observe God's Law.
Contrary to popular beleif, the Festivals and Sabbath existed before the Sinai covenant was given to the Children of Israel (including the Jews and a multitude of gentiles that had attached themselves to Israel when they left Egypt).
On the 4th day of creation, the Festivals were ordained:Genesis 1:14-19 GNB Then God commanded, "Let lights appear in the sky to separate day from night and to show the time when days, years, and religious festivals begin; (15) they will shine in the sky to give light to the earth"---and it was done. (16) So God made the two larger lights, the sun to rule over the day and the moon to rule over the night; he also made the stars. (17) He placed the lights in the sky to shine on the earth, (18) to rule over the day and the night, and to separate light from darkness. And God was pleased with what he saw. (19) Evening passed and morning came---that was the fourth day.
On the 7th day the Sabbath was ordained:
Genesis 2:1-3 ESV Thus the heavens and the earth were finished, and all the host of them. (2) And on the seventh day God finished his work that he had done, and he rested on the seventh day from all his work that he had done. (3) So God blessed the seventh day and made it holy, because on it God rested from all his work that he had done in creation.
Edit: Easter is NOT Passover:
http://www.coghomeschool.org/site/cog_archives/booklets/THE%20PLAIN%20TRUTH%20ABOUT%20EASTER.htm
Edit: Anyone who thinks Christians are not Israelites or descendents of Abraham have not really studied the subject. We are both. Some of us in the spiritual sense only, some in both a physical descent and spiritual sense.
Buddy, above, is a good example.