When I taught we had a 3 party limit. The class decided which occasions we would have our parties and then we commemorated the other occasions in different ways. I prefered the Holidays we didn't have "parties". We cooked, did art, writing projects, etc. around the holiday theme. Even Halloween costumes can be part of a lesson.
The only party that I really felt was worthwhile (from a teaching standpoint) every year was Valentine's Day.
My kid's school has parties for everything. Every party seems to involve eating the kids eating until they are sick and nothing else. I think it's pretty easy for the teachers to think "we're having a party so I can't waste any more time on the holiday". I'd much rather see special art projects, etc. coming home than have them watch a movie and eat.
I just want to add before I get flamed for being a spoilsport - I don't remember a single party in grade school (although I know we most have had some). However, I do remember many of the holiday art projects etc. we did and the fun we had doing them.