Dennis Prager (emphasis added):

One is that the left sees in Christianity its primary ideological and political enemy. And it is right to do so. The only large-scale organized opposition to the left comes from the traditional Christian community — evangelical Protestants, traditional Catholics and faithful Mormons — and Orthodox Jews. Leftism is a secular religion, and it deems all other religions immoral and false.

From Karl Marx to Vladimir Lenin to George Soros, the left has regarded religion in general and Christianity in particular as the “opiate of the masses” — a drug that dulls the masses into accepting their oppressed condition and, thereby, keeps them from engaging in revolution.

The left understands that the more people believe in Christianity (and Judaism), the less chance the left has to gain power. The left doesn’t concern itself with Islam, because it perceives Islam as an ally in its war against Western civilization, and because leftists do not have the courage to confront Islam. They know that confronting religious Muslims can be fatal, whereas confronting religious Christians entails no risks.

Second, the left regards Christianity in America as an intrinsic part of American national identity — an identity it wishes to erode in favor of a “world citizen” identity. The left has not only warred against Christmas; it has sought to undermine other national identity holidays. For any number of reasons, not only including the left, Americans no longer celebrate George Washington’s birthday (it has de facto been replaced by the utterly meaningless “Presidents Day”) or Abraham Lincoln’s birthday, as they did when I was a child, my father was a child and his father was a child. The only American celebrated in a national holiday is Martin Luther King Jr., which is acceptable to the left since he is not white. One proof of the left’s desire to undermine specifically American national holidays is its war on the two remaining specifically American holidays: July Fourth and Thanksgiving.

The left deems Thanksgiving a historical fraud and an immoral celebration of “genocide” of the American Indians — which is what American children are now taught in many American public schools. And “happy Thanksgiving” has been replaced by “happy holidays.” As for July Fourth, The New York Times is leading the undermining of the celebration of America’s birthday by declaring that the real founding of America was 1619, the year, The Times asserts, African slaves first arrived on the American continent.

Of course, there is still Veterans Day and Memorial Day, but they are not specifically American national holidays; just about every country has such holidays.

But Christmas is a problem for the left. It celebrates religion, and it does so in quintessentially American ways (take American Christmas music, for example).

The third and final reason is that the left is joyless. Whatever and whomever the left influences has less joy in life. I have met happy and unhappy liberals, and happy and unhappy conservatives, but I’ve never encountered a happy leftist. And the further left you go, the more angry and unhappy the people you will encounter. Happy women and happy blacks, for example, are far more likely to be conservative than on the left.

Christmas is just too happy for the left. “Holly, jolly” is not a left-wing term.