I think that one of the reasons that Skyrim (and the entire ES series) does so well is that it has always been an SP game and is designed to be that, and that alone. Despite it's problems, it offers a
huge amount of content which is key to making a good SP game. Let's face it, Skyrim is a freakin' whole country in a box, and that is a bar set by Bethesda that many other developers can't, or don't want to, match.
If you've been keeping up with the gaming industry news, more and more of the top people are starting to admit that "content is king" when it comes to games. People no longer are happy with a few hours of fast and furious gameplay in SP, and then forced to move to MP for a continued experience. That, and the fact that many MP games are riddled with problems (bots, cheats, vulgarity/racism by users in chat, etc.), has left a lot of people with a bad taste (and that's not mentioning the lack of the developers' bringing anything new to the table to keep it fresh and exciting).
There are also the facts that the gaming landscape changes regularly, releasing new IP is risky in a downturned environment, and investors want to be kept happy and know their investments are safe (cue DRM).
These, however, are the same risks any company faces when doing business and in my mind it's just pointing fingers at others for failure to make truly compelling products that sell well.
Okay, off my soapbox.