Because the combat in DD requires constant teamwork and synergy between you and your pawns. Skyrim's partner AI consists of following you, and attacking things that threaten it. It's not exactly robust, and the whole world is balanced to be played solo. I don't need to rely on my partner to get things done. I can dismiss them all and I have total control over my situation. That simply isn't the case with DD. You rely on the spells and abilities of your teammates to take down large creatures. The demo shows that that much is obvious.
I don't know why you guys are all getting rabid over something I'm saying. The game WOULD be better with co-op. You don't have to agree with me, and if you don't, that's super cool, but my opinion still stands: the lack of co-op is probably why I won't buy the game. I don't like having to rely on AI to get specific things done (unless I can take over direct control, which I cannot).
You can't be sure of that, and that's the crux of the matter. Combat aside, they'd have to completely change how the story/quest stuff works to accommodate a meaningful co-op experience. Otherwise it would just be something basic like letting your friends drop in and take control of your pawns. All progress would be tied to your specific game, and your friends would just be along for the ride.
Anyway, we can't be sure how the game would have turned out if it was designed with co-op in mind. I do know, however, that Capcom's previous attempts at this (Resident Evil 5 and Lost Planet 2, and hell, the entire Monster Hunter series) didn't appeal to me at all, so I have reason to believe that Dragon's Dogma wouldn't either had they pursued the co-op path. The fact that they worked hard to make a great single-player experience is one of the things that has me so interested in this game.
It's unfortunate that you don't want to buy it because of this, but I'm not going to let it bother me. I just hope you realize that your opinion that co-op would make the game better is just that: your opinion. Stating it as if it's a fact doesn't change that it's simply your opinion.
A LOT of people just assumed it would be co-op
As did I when I first saw it. I felt sad, because I thought the game looked awesome, but I wouldn't be able to fully enjoy it at my leisure. It wasn't until I found out that those pawns were all AI controlled and they had designed the game to be a single player experience from the ground up that I got excited. I could rest at ease knowing it wouldn't be another bullshit compromised single-player experience like Resident Evil 5 and Lost Planet 2.