It was a good variety of factors. Big, splashy multiplexes were popping up more and ticket prices were starting to rise considerably, the 9/11 attacks made people want escapist entertainment at a time when streaming didn't exist, and all those movies had tons going for them in the advertising, whether it be the people behind them (Monsters, Ocean's) or the iconic source material (Potter, Fellowship).
There was a really good thread talking about this stuff about 5 years ago, which really crystallized the absurd bump in attendance in 2000 and 2001.