Several Microsoft watchers have asked that same question. The concensus answer seems to be, "the board couldn't think of anyone to replace him".

Now go read the links in my original reply and see why the VI article is flawed.

>Ballmer is 4 to 6 years overdue to be tossed. How has this guy held onto his job?
>
>Brilliant people doing mediocre work really says it all.