Capacity is 75,000. I'm not sure why it sells out so incredibly fast. University of Texas is in Austin which could explain some of it. 50,000 students are enrolled there and Austin is a very music friendly city so I think it's a combination of the two. Lots of early twentysomethings in a musical city. There is a good amount of people 35 and older that attend each year as well. More so than any other major festival I've been to. Also, ACL is one of the biggest festivals in the US and since C3 took it over they've provided it with a lineup fairly similar to Lollapalooza each year. So I think it's a combination of the lineup and location that help it sell out so fast.
Been to ACL like 3 times. Demographic definitely skews older than coachella. People bring lawn chairs. I've seen some seriously great shit there but when it grew to 75k it was just too much for me.