At this point I think it's largely a lack of viable alternatives. Despite increasingly loud noises of complaint from Windows users, it remains the only reasonable thing for a vendor to preinstall on a computer. OSX only comes on (expensive) Macs, and they tried Linux on netbooks back in 2007 and it was a disaster (partly due to vendor fragmentation, partly due to marketing that papered over the whole "different OS" issue).

We so nearly had it with Ubuntu. I suspect sooner or later Google will pull it off in some form or another, if ChromeOS doesn't already count.