Here are some of the reasons why PC gaming is less popular in the U.S. than console gaming or at least reasons that I can think of:
1. In order to play PC games you need to have some minimum degree of computer literacy. As computers have become increasingly mainstream the percentage of the computer population that is composed of geeks has decreased. Many people wouldn't know what to do the minute they found out that they needed to install a driver.
2. In order to play PC games, at least first person type games, you need to either buy an expensive pre-built gaming rig or have some knowledge of computer hardware. The percentage of people with knowledge of computer hardware is probably less than the percentage that is computer literate. Is that $500 rig you bought at Big Box Store really going to run Crysis? What if it's necessary to (God forbid) open up the case and install RAM and a video card. And what happens if it's necessary to upgrade the power supply and how the heck would your average computer user know that he needs to upgrade the power supply in order to use his new video card. Who wants to put up with all of that hassle anyway?
3. It's possible that many PC gamers, at least myself, tend to latch on to one or two of their favorite games and play them almost exclusively which means that they won't feel a need to buy new titles anytime soon--your WoW players for example or your hardcore Unreal Tournament fans who avidly play a game that was released in 1999 (or 2004 in the case of UT 2004) to this day. Some PC games are just so good and so engrossing and offer so much custom content that they end up eating into sales. I buy an average or one-and-a-half games a year and mostly end up gravitating back to my favorite games (which is, of course, Unreal Tournament). Personally, I'm only interested in games of the highest game play quality and production value which might also be true for many other PC gamers. (Put a great big X through 95% of all the crap that's out there.)
4. Many PC gamers take pride in pirating games. (These guys aren't going to keep games on stores' shelves.)
5. Only a small percentage of non-MMORPG players seem to be at all interested in the real advantages of PC gaming--free downloadable user-made custom content, online multiplayer gaming, voice comm, game discussion forums, and Internet Relay Chat (IRC) or XFire. Most console gamers have no concept of the big advantages of PC gaming at all or perhaps they just don't have any interest in that. They truly have no idea what they are missing.