There are multiple reasons why the crowd isn't as raucous as it used to be.
Finacial reasons. It is tougher than ever to buy an affordable ticket and older more affluent people generally aint as loud as younger ones. As an older guy, i can attest that i am not as drunk or as loud as I was in the 90s. Well... not most games
Scheduling. Those 12:00 and 3:30 September games are f...ing miserable due to the heat. And referencing the above point about an older crowd just adds to it.
Sterling blaring music until the 15 second mark on the play clock takes the crowd out of it as well, IMO.