I keep in touch with a few friends from HS. But by in touch, I mean I see them a few times a year, I don't hang out with them regularly.
I was one of the few of my friends to actually go to a 4 year college, most went to the local community college. Not because they were stupid, mostly because they were lazy or not ready.
I now live about 250 miles / 5 hours away from where I grew up so its not like I can just call them up one night and go hang out.
Not to mention college changes people quite a bit. You make new friends there, you friends back home go on with their lives. Each group has their inside jokes and after a while you don't know the inside jokes of your old friends anymore. So you begin to lose touch.