I think there are several reasons.
1) Schrinking PC gaming market, this amongst other reasons due to piracy, inefficiency and incompatibilities. Being uncomfortable (for example no standard wireless gamepad, bad support for multiple controller gaming)
2) Lucrative console market (PS1 selling over 100 million units).
3) Advances in home cinema technology, past SDTV vs monitor (advantage PC monitor), today HDTV vs monitor (advantage usually HDTV for gaming and crisp movie watching). Great surround sound setups. Broadband internet has become wireless, easily usable from anywhere in the home without ugly cabling.
4) Consoles can leadfrog with technology without having to make sacrifices for extensive legacy support (old hardware and software, on a console due to long lifecycles this can be taken care of through emulation). For example, millions of consoles all using Blu-Ray and a super fast 8 processor configuration.
5) PC gamers are harder to make money from, no hardcore PC gamer would accept to pay extra monthly or yearly just to play a game online. It's near impossible to introduce something like that when people are used to better (online gaming is relative new for consoles).
There are many other reasons, I think Microsoft sees it in their best interest to slow technology advancements as much as possible. For example x86 is an awful architecture, the jump to more pleasant and powerful CPU architectures is way overdue. But leaving x86 behind would mean a lot of legacy software support would be dropped and Microsoft's monopoly is entirely built around this. However if the same amount of R&D would be spend on advancing better foundating technology, we would see a lot more progress (more engergy efficient, smaller, more powerful and cheaper to make).
Just imagine if the entire gaming industry would have gone full force supporting the Cell and Blu-Ray, that could have been a near deathblow to Microsoft gaming. Now multi-platform games will be very similar on 360/PS3/PC, the Wii is probably less relevant as such technology is not a threat to traditional PC gaming that much. A PS3 can already run a fully featured OS pretty decently (running on just one of its 8 CPUs and lacking direct GPU driver support, utilizing the Cell with awesome results for newly written scientific tools ), who knows maybe in the future a PC partner would get the idea to build a PS4 into a full PC rivalling computer some day.







