Here are the reasons:
1. in the FSU, fir tree had absolutely zero religious attribute.
2. In most households, the fir tree is setup after December 25th
3. Fir tree is called a New Year tree not a X-Mas tree, just as Santa Claus is called Grandpa Frost
4. New Year was, and still is, easily, the most joyous of all FSU holidays as it wasn't attached to any communist state holiday
5. Orthodox XMas is in January, not it December. Orthodox Church has not used any association of XMas and th New Year tree.
6. Children love the New Year tree, it creates a very festive, magical ambiance in a home, celebrating the New Year. The presents are found under the tree on January 1st, not on December 25th.
7. For most Jews from the FSU, The New Year tree, devoid of any religious bearings, is much more innocuous than the much anticipated Haloween( widely celebrated by American Jewry).