Why don't Christians celebrate the feast of tabernacles?

Moral laws in Old Testament are always the same (For Example, Ten Commandments and Other Moral commandments such as Deuteronomy 6:5, Leviticus 19:18, etc).

Unlike Moral laws, Ceremonial laws were written specifically for Hebrews (or ancient Israelites) due to the fact that God freed Hebrews from Egypt. This doesn't apply to Gentiles.

Feast of Tabernacles belongs in this category, because Feast of Tabernacles was celebrated as a remembrance that the people of Israel lived in booths when God brought them out of the land of Egypt.

Leviticus 23:42-43 (ESV)- "You shall dwell in booths for seven days. All native Israelites shall dwell in booths, that your generations may know that I made the people of Israel dwell in booths when I brought them out of the land of Egypt: I am the Lord your God.”

Salvation came to the gentiles through the arrival of Jesus Christ on earth and through his sacrifice for us. Through his sacrifice for us, the ceremonial laws (which includes all sacrifices) were completely fulfilled.

We only have to keep the moral laws commanded by God and his son Jesus Christ.

Why don’t Christians generally celebrate Tabernacles now?

The simplest answer is that Christians never agreed to observe it. The Israelites agreed to keep the Law of Moses at Sinai by confirming an oath in a bilateral covenant.¹

⁷ Then he took the Book of the Covenant and read in the hearing of the people. And they said, “All that the LORD has said we will do, and be obedient.” ^{NKJV, 1982}

Even if one is a Jewish Christian (i.e., a Christian by faith who is of Jewish descent by birth), they died to the Law by their union with the Messiah.²

⁴ Therefore, my brethren, you also have become dead to the law through the body of Christ, that you may be married to another—to Him who was raised from the dead, that we should bear fruit to God. ^{NKJV, 1982}

The rabbis understood that “as soon as a man dies, he is made free from the commandments.³ Since we have died with Christ,⁴ we are free from the commandments (this would pertain to those Jews who were formerly covenantally bound to observe the Law of Moses).

In summary:

• Gentile Christians never agreed to observe the Feast of Tabernacles in a bilateral covenant; therefore, they were never obligated to observe it.
• Jewish Christians were obligated to observe it, but because they are dead with Christ, they are made free from the commandments and are no longer obligated to observe it.

Footnotes

¹ Exo. 24:7
² Rom. 7:4
³ Babylonian Talmud, Seder Tohorot, Tractate Nidda, Chapter 8, Folio 61b, Gemara: כיון שמת אדם נעשה חפשי מן המצות
⁴ Rom. 6:8

The prophecy has yet come to pass. I wouldn't say the current period is anomalous at all. Up until now, the feast has been celebrated by Jews only (ok, a few Christians). That's a very small minority of people. The prophecy talks about all nations, which is something very different from history.

You mention Jesus going to the feast in John 7, but notice what Jesus says about the feast:

John 7:8 (ESV)
⁸ You go up to the feast. I am not going up to this feast, for my time has not yet fully come."

Granted, Jesus eventually went in secret. But it doesn't seem to bother anyone that Jesus would not go. If it wasn't necessary for even Jesus to go, why should we, gentiles?

Orthodox tradition holds that the current Feast of the Transfiguration of Christ replaces the Hebrew Feast of Booths, and we are a significant minority, if not a majority, in many countries.

In this guise, one could interpret this passage to mean that those who do not celebrate the Transfiguration, i.e. through Christ become transfigured themselves, in That Day will experience severe suffering.