That was the case until about two or so years ago; then the market exploded, and now you can get very fast broadband anywhere in Japan for cheaper than the US. This has set off a new market for local online games, although it still isn't quite as big as it is in the rest of eastern Asia.
As for the PC market, you could attribute it to cultural differences. For most people in Japan, their introduction to home gaming was via the Famicom. Over here, more likely than not, it was through one type of computer or another. Even so, there has been a real, non-porno PC gaming scene since the early 80s, and there's one monthly magazine in Japan (Login) devoted to covering all the non-porno stuff.