First, because each group thinks they know exactly why the other group feels the way they do.
"She is silly and immature to get so excited about something like a strand of colored lights."
"He has a hateful heart because he doesn't get any joy from a holiday that prizes family time."
Both of them have made unfair judgments.
I live with a Grinch. He isn't hateful. He actually enjoys the day, especially when our children were young and now when our grandchildren can be with us. His "Grinch-ness" comes from the commercialization of the season. To him it's about retail trying to spread the time out longer and longer--yes, even into September—so they can make more money. I can see that, and having done a little sales in my years, I know he is probably right. Recognizing that issue and disliking it does not make him the Devil Incarnate.
As for the Whos, if they have happy memories of childhood and want to recreate those memories for their children and grandchildren, or even recreate it for themselves every year, I am happy they were so blessed. That does not make them silly or immature.
And so, as with every other disagreement, the issue is about each side making sweeping judgments and generalizations they should not make. You don't make assumptions about motives, for one thing, and you don't stoop to name calling for another.
Yes, even the pro-holiday crowd (the ones who say it is all about joy) is guilty of those things. They are the proverbial pot calling the kettle black. Maybe they have lived a sheltered life. Maybe they are just so self-oriented they cannot imagine a reason to think differently than they do. But the truth of the matter is that some people do not like the holidays because they once endured a tragedy during that season and the anniversary is too hard to bear. Some people have not had the blessing of a close and loving family, or they have no family left, and so all these commercials showing happy families together just rub their noses in their own loss and loneliness. There is a reason that suicides are common in December. You would think that a group like the Whos, who consider themselves "the good guys," would empathize instead of criticize by calling the other group a bunch of meanies, or, as we are putting it here, Grinches, or some other epithet. See what I mean about pots and kettles?
I am, alas, not surprised to find such a rift even among Christians. If we don't have a doctrine to argue about, or politics to take each other to task over, we will find something—even a civil holiday supposedly about joy and peace. It's time to stop judging each other, as Paul told the Romans about something far more important. Do your thing quietly and let the other guy alone. God loves you both—whether you like it or not.
Who are you to pass judgment on the servant of another? It is before his own master that he stands or falls. And he will be upheld, for the Lord is able to make him stand. One person esteems one day as better than another, while another esteems all days alike. Each one should be fully convinced in his own mind. The one who observes the day, observes it in honor of the Lord. The one who eats, eats in honor of the Lord, since he gives thanks to God, while the one who abstains, abstains in honor of the Lord and gives thanks to God. For none of us lives to himself, and none of us dies to himself For if we live, we live to the Lord, and if we die, we die to the Lord. So then, whether we live or whether we die, we are the Lord's. Rom 14:4-8