Along these things, in all seriousness, I'm surprised sports wasn't in the top 3 reasons people mentioned.
Like anything else, this is an evolution. While many Catholics might not attend Mass as much, it doesn't necessarily reflect a dive in Catholic principles either. There are many, many things that a Catholic education is good for, and to point the finger at Mass statistics doesn't really tell the whole story.
I'd argue more that it's declined because of the lack of faith in the leadership of the Catholic hierarchy. As in many walks of life, they felt taken advantage of by the people at the top because of lies and cover-ups. If you can't trust it, it makes sense you're going to lose a following. In saying that, the congregation of parishes still do many, many things to ensure a sense of positive community.