I feel like, being born in 1978, I grew up very in tune with the rise of video games. Flying my X-Wing through the Death Star trenches on the ATARI, felling foes in Golden Axe on the Genesis, getting seriously frustrated with Battletoads on the NES, and falling in love with the original M&B on my old PC. One thing I have noticed over the years is that AI is lame. Its better, don't get me wrong, but still lame.
I have begun so many SP M&B games and never once had an interest in actually finishing any of them. Why? Stupid lords besieging a castle for weeks, running off from a major campaign to go enjoy some butter, enemy lords with a whole damn army running through the apparently empty kingdom with no one to stop them just to go get your one, hard fought castle as a new fledgling lord. Its just boring, and predictable.
Then I finally moved out of the boondocks and got internet and found CRPG years ago. (Town of 64 people) Loved it. Crazy people doing crazy things. Unpredictable. Sure there are naked 2H spam lovers running everywhere but thats some of the magic. I haven't been able to go back to SP since then. Its just too hard to get immersed into something when you know whats going to happen.
And that is why I think there is so much interest in COOP. You get the immersion of SP with the randomness of MP when someone else does something totally unexpected and bat **** loony.