because there was no alternative at the time and once a platform gains momentum it's hard to challenge. just look at ms trying to take on ios and android in the mobile sphere. plus, do you really want to get into all the anti-competitive behavior ms has been charged.
heck even look at this: "The Justice Department will no longer enforce restrictions that bar the company from pressuring computer makers not to promote competing software products such as browsers or media players. This includes the design of the next version of the operating software, now in early development. Houck told the judge March 7 that the states are reviewing early versions." (http://www.bloomberg.com/apps/news?pid=newsarchive&sid=aZlZaQM5xGZg)
So basically it's even ok for ms to literally pressure oems to not put anything but their os on their systems