Originally Posted by -King-
Bigger arenas? Expansion teams?
They expanded a bit, but that's not the only reason for the increase. With the exception of a few teams, games aren't typically sold out every time, so the increase is a combination of more games and higher attendance.
Here's the source I found BTW.
In terms of per-game attendance, it went up from about 16.5k per game in the late 90's to about 17.5k/game in recent years.