It is simplified, but generally speaking these are main possible paths with default settings.

X axis is market share (with some guessing).

I know a small examples of renderings could be included for better understanding, and I will post this & other refinements later.

A few things not mentioned:
Linux with FreeType rendering, but I have no idea how many people use subpixel rendering or other settings. Each setting has probably less than 1% of users.
Quartz rendering does also other types than subpixel AA, but I would say they are in minority (I really don't know).
There is also an old version of Safari, with Quartz rendering on Windows with default setting.

According to Dick Brass, a former Microsoft VP, it was internal political sabotage:

“[O]ther Microsoft groups… felt threatened by our success.