A Christmas tree ordinarily in the US is an evergreen. There is nothing inherently wrong with having one in your house or outside of your house. It is unlikely that early Christians in Israel had pine trees in their homes on Christmas, since those trees are not numerous in the Holy Land.
But contemporary Christians attribute religious significance to a Christmas tree, and most rabbis will discourage or forbid Jews from having them in their homes. In addition for the average Jew, there is a psychological barrier to having a Christmas tree.
And we have the latest incredible credible 32% statistics from the Pew study of Jews in America, "A Portrait of Jewish Americans". One number that jumps out for me is that 4% of modern Orthodox Jews have Christmas trees.
Jewlicious picked out the salient quote from the study:
Page 80: About a third of Jews (32%) say they had a Christmas tree in their home last year, including 27% of Jews by religion and 51% of Jews of no religion. Erecting a Christmas tree is especially common among Jews who are married to non-Jews; 71% of this group says they put up a tree last year. Compared with younger Jews, those 65 and older are somewhat less likely to have had a Christmas tree last year. And relatively few Orthodox Jews, including just 1% of UltraOrthodox Jews (and 4% of Modern Orthodox), say there was a Christmas tree in their home last year. Attending non-Jewish religious services is an infrequent occurrence for U.S. Jews; just 15% say they do this at least a few times a year.