I think because it changes so much as you grow up. Christmas, for me, has always been about togetherness and about traditions - there has to be certain music and certain food and certain activities, and they're always the same. It was never about presents for me, at least not since I was very very young. Thanksgiving, always a nice meal with the family. Doesn't change. Easter, same deal, plus the added bonus of chocolate.
Halloween, though? Different all the time. As a kid, you've got to have the perfect costume. You hope for perfect weather to go trick-or-treating, which of course hardly ever happens, and suddenly your perfect costume is ruined by those mittens you have to wear... You have the obligatory 'dump out all your candy and trade with your friends.' It's such a happy holiday, but can be so easily ruined - rainstorm on the wrong night, you get all orange suckers and no one wants to trade their reds and purples for them - and bam, you're in tears.
Then you hit junior high, and all of a sudden you've got to deal with the preteen angst of being way too cool for this shit. Of course, if you're my friends and I, you don't care and go out anyway, and listen to all the adults say "aren't you a little old for this?"
By the time you get to high school, costumes are cool again, and you can totally have Halloween parties instead of trick-or-treating. But there's still the issue of finding the right costume - original, but not too difficult to make/find, and recognizable - nothing worse than having everyone ask what you're supposed to be. And then you need to either get invited to a party or host a party - so complicated.
And then you hit university, and everyone uses Halloween as an excuse to get trashed and wear as little clothing as possible. Which is sort of awful. The past two years, I haven't been such a big fan of the holiday. Last year, for example. I went to a really awkward Halloween party, which is another story altogether, but then had to work overnight. This involved rescuing about 20 scantily-clad girls who had, in their drunken stupor, locked themselves out of their rooms, and almost needing to clean up puke. Not cool.
This year, I decided I didn't care that much about Halloween and wasn't going out just for the sake of it. My original plans were to stay in and watch a couple Halloween movies. But I ended up going out to a bar with my roommate to hear a jazz/funk band that we adore (the Mello-tones), and had a fabulous time dancing our hearts out. It was the first time in a while that I actually really enjoyed Halloween.
And from our observations last night, it seems like the 30-40 crowd is stuck taking their kids out trick-or-treating, and then when you hit 50ish, you get to go back to going out and dancing like crazy.
So why is Halloween this way? What's its deal? Why is it so volatile? I guess maybe partly because it's not as family-oriented as most holidays but more of a social affair? I don't know, but it's strange.