Why don't Christians celebrate the feast of tabernacles? Although there are some who celebrate the feast of Tabernacles they are definitely in a minority.

A prophecy in Zechariah speaks of a time when all nations will have to keep the feast:


  Zech 14:16 And it shall come to pass, that every one that is left of all the nations which came against Jerusalem shall even go up from year to year to worship the King, the Lord of hosts, and to keep the feast of tabernacles. And it shall be, that whoso will not come up of all the families of the earth unto Jerusalem to worship the King, the Lord of hosts, even upon them shall be no rain. 18 And if the family of Egypt go not up, and come not, that have no rain; there shall be the plague, wherewith the Lord will smite the heathen that come not up to keep the feast of tabernacles


We know that Jesus celebrated it (John 7:2) and if we believe the prophecies then we will keep the feast again in the future. The current period of not celebrating appears anomalous.

Why don't Christians generally celebrate Tabernacles now?
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?