Surely you realize that for some people, 12 hours or so of togetherness with people is enough. (Cause I'm sure if there's an after party, there was also the mandatory pre-wedding manicure and hair and brunch and....) And other people just get TIRED by midnight. And some just don't like parties, regardless of who is there. And others may very well be up to their ears in wedding by the time the reception is over and just want to get away from Bridezilla. And some may have husbands or kids that they would rather spend time with. And some may have a 4am flight out of town because they rearanged their schedules to ensure they could be there for the wedding, but now have to get back to work to do xyz....
There are a million zillion reasons why someone doesn't go to the after party, including just that THEY DON'T WANT TO.
As I said above, my sister had an after party at her wedding. I love my sister dearly. I would do almost anything for her and on her wedding day I wanted nothing more than to make her happy. But after the reception, I was DONE. There was no single teeny bit of me that wanted to keep dancing and drinking. We had already been doing that for four hours at her reception. I wanted to go to bed. I forced myself to go for a little bit, but no - I didn't at all want to join the party. I'm super thankful that she didn't reconsider her choice of a bridesmaid just because I didn't want to go out drinking with her and her college buddies.