1)They want to have an 'adult' party where everyone can drink and have fun without worrying about kids running around and guests needing to leave because the kids are getting cranky
2)To save money. Kids rarely eat the adult dinner and paying for them is extra money, many don't want to spend
3)Some don't want crying screaming kids 'ruining' their magical day
Of course if you're a parent this means you will have to find a sitter if you intend to go, which in many cases will cost you more money on top of travel, clothes/hair/make-up, and wedding gifts for the bride and groom. Also it might seem rude to you, because your children are a part of your, and presumably the bride and grooms family, so why aren't they invited as well?