I just checked to look, and it says 300 is playing at 11:15 I remember to The Mummy Returns at 11:30 and Mr. Deeds and 11:45.
AMC ususally shows late showings on Fridays/Saturdays, I remembered I watched "Employee of the Month" last Oct at a 1230am showing and ended like a little past 2am.
Its very simple, not enough people go! Highland is about the most centrally located theatre and the most ghetto theatre in town. If more than 50 people would go, the theatre would stay open another round. The AMC at barton creek stays open later whenever they get booked for product that will perform otherwise they will close 1045ish.