Just go with me on this.
Firstly, I've always believed Treyarch has put a little extra something into their Call of Duty campaigns, at least in direct comparison to Infinity Ward. World at War and Black Ops both had very enjoyable single-player adventures in my mind, and despite the PR and pub the "No Russian" sequence got in MW2, I still say Treyarch excels in the campaign area.
Secondly, the concepts, themes, and topics that will apparently be tackled in Black Ops II are pretty freakin' massive. If you read up on what we know thus far, it seems like the developers are really pushing to deliver a well-paced and surprisingly in-depth plot. On top of which, if you examine the trailers and snippets of gameplay we've seen, it also seems clear that a wide variety of mechanics will play a role in that campaign. If you factor in the diverse locations as well, you're looking at something that couldn't possibly be finished in only four or five hours…right? Ten seems more reasonable.
Thirdly and lastly, I really do believe those who enjoy single-player experiences have made their voices heard. For the most part, they've been drowned out by the legions of multiplayer fanatics who only play CoD for one reason, and that's perfectly fine, considering multiplayer is indeed the franchise's bread and butter. But I know some gamers were unimpressed with the MW3 campaign and I believe the disappointed were even more vocal this time around. Activision may not have cared to listen but I bet some of the guys at Treyarch noticed, especially because they do pride themselves on the campaigns they produce.
Oh, and of course, Activision is dumping a ton of money into the new production. And money can buy talent; they've got "The Dark Knight Rises" co-writer David S. Goyer and composer Trent Reznor, for instance, and you can expect more big names to come to light as the year progresses. I will say that an old engine can still hold Treyarch back but as I've said in the past, they've done some pretty amazing things with that old clunker.