there sure are a lot of people try to fuck up windows. It's much less of a problem on other operating systems.
Possibly because Windows has many, many, many more users than other operating systems do.
It's like the old urban legend about bank robber Willie Sutton (http://en.wikipedia.org/wiki/Willie_Sutton) answering a reporter's query about why he robbed banks with "because that's where the money is." Crapware vendors focus on Windows because that's where the users are.
It's not advantageous for a parasite to immediately kill it's host. By 'wiped out' I don't mean the literal death of windows installs, I mean in the sense that a pristine windows install will, on average, slowly accumulate cruft until it's on the verge of being unusable. It's not a 1:1 analogy. In a strictly practical sense, an individual windows install may not be nonoperational due to mal/spy/crapware, but the quality of the user experience goes way down as a result of it.