My thoughts on this, and I have no background in programming or game design so it's purely speculation:

I think part of the explanation for this is development time and efficiency. While technology continues to improve year after year, a game along the lines of Skyrim is going to sit in development for multiple years before release. That would prevent it from taking full advantage of technological advancements during the middle and end of its development. On the other hand, in the case of the COD franchise, it seems like those games get released at a pretty steady rate. I would wager there's a lot of recycling and modifying of the previous technology to push it a little farther than it went before (think games initially released on any given console vs. games released several years later for the same console).

Basically, I think it just comes down to how much time and money the developer has to work on a game. There's probably some corporate politics that come into play with this as well. Companies like EA, Blizzard, etc. are more likely to have the resources to push the envelope. Corporations of that size, unfortunately, are going to be more concerned with keeping the bottom line of a project in the black than breaking any boundaries in visual content.