@Dare: Because MS was the first to market with
a reasonably good, reasonably stable, and inexpensive computer way, way, back in the day. Because it was all of those things plus the hardware got even more affordable, they developed the biggest installed base. That installed base is their most valuable asset. If you move away from that base you encounter friction when you deal with other businesses. Whether it's the act of explaining to them that there is nothing wrong with your documents when they open them and get the "converting" message (because MS has tweaked something and even though your FOSS program supports the format MS said was public it doesn't exactly any more), or outright rejection because you aren't using the right software it is still friction. In business, the less friction on things that don't matter, the easier it is to deal with friction where they do.
Not that I'm necessarily happy about this, but it is at the heart of the matter. In fact, this truth even causes MS problems: Do you really think MS extended support for XP for as long as they did because they wanted to? Do you really think they want businesses to still be stuck on IE6? Whatever you thing of the quality of their new OS and browser, MS obviously want people buying new stuff because that maintains or increases the MS money stream.