Well, it's easy to answer the question of the mainline protestants - but very hard to answer the deeper question of 'what now?'
The question of the mainline protestants is best answered by looking at their most ill-fated representative, the universalist/unitarians, formerly known as congregationalists. They originally relied on a sort of mob rule to perform 'church discipline' - but they quickly found themselves on the road to the lowest common denominator when it came to orthodoxy and orthopraxis. Every standard became hollow. Community even became hollow, with no demands placed on the individual. Social justice became another word for 'being nice to people in a thoroughly unobjectionable fashion' and redemption lost its core as suddenly nobody had anything to be redeemed from. People were even told that the things they didn't like about themselves, they should accept.
'People' saw through this. They continue to see through it. Offering nothing - with the occaisional big argument over nothing - the universalist/unitarians even declared in the early part of last century that they weren't a church. They lost most of their members and quickly backed away, deciding to call themselves churches again, just to keep people with a christmas and easter minimalism in the fold.
Now, the problem for the evangelicals is the temptation to try to get everybody 'through the door' and 'praying the prayer' and 'born again.' When your sole reason for being (purpose of purpose driven, like Rick Warren) is getting as many people through a door, the temptation is to multiply the number of doors, move it forward, make it larger, and double count people who come in, go out and come back in again...
All of these are supported by the theology of assurance - once saved always saved; so just getting that one little prayer becomes a one-way trap, a sort of religious roach motel. No matter that the person goes on to practice buddhism... still saved.
This has the problematic effect of watering down community, making it hard to tell what being 'saved' really means, etc. It means evangelicals following the mainstream denominations in their own arc of irrelevance and getting another group chasing in their tail. Ironically, it seems that one of the groups chasing them is the orthodox neo-catholics.
The pentacostals also had their own arc in all this, peaking it seems in the 70s and 80s, before folding into the evangelicals to a degree.
It can all be a bit mind boggling; and yes, largely it doesn't make much sense. Alot of the problem is that organizations never die, they just fade and fade and fade. Splits stay, unity never seems to return. But who knows.