I think it's because they are in an "innovate - iterate" cycle, and they are incompetent at getting things right the first time they try them. Partly due to their own shortsightedness, and sometimes due to third party hardware and software vendors
95 - innovate - was seen as a good release when it came out, because innovation was needed - but in reality, it was a steaming pile of unreliability
98 - iterate - improved stability from 95 - was a good OS (by the time we got SE anyway)
{ ME - innovate - sucked major ass for no good reason
{ 2000 - innovate - good release but lack of drivers and compatibility held it back
XP - iterated on 2000, bringing it into the home, and probably their most successful product ever
Vista - innovate - new kernel and driver model - sucky reception, basically due to the ecosystem not being ready for the changes
7 - iterated on Vista - home run, straight out of the park - they didn't try to do anything too shocking here, just delivered a good product
8 - innovate - let's re-write the book on UI - flop
9 - iterate on 8 - I actually expect it to be a good release
/my theory