I don't know about the 90's being the last great decade. But the reason for the slide in ratings for movies is largely due to the decline in cinema attendance. The segment of the population who spends the most on movie going now, are young teens. So Hollywood frequently softens their ratings to sell more tickets.
I was pretty surprised when the first AvP movie from 2004 was rated PG-13, when every other alien and predator movie were all rated R.
By the time John McClain's trade yippee kai yay line got cut off in the last Die Hard in 2007, I was just like... damn teenie boppers and their movie going disposable income.
2001 was the year reality TV got big, and the year we finally woke up to something to be afraid of again since the cold war - global terrorism. I blame both for really sending entertainment and creative expression downhill.