Because quicktime already has an established user base. It is a proven format and has been around for several years. Previously it was the only format that could handle moderate quality video previews and trailers at a managable size when many people where still on dial up modems... so 63 meg files where a bit out of the question and still are in some respects. Not everyone has the luxory of a decent connection.
And at the time windows media player was a primative piece of junk that couldn't handle many compressed formats.
Mpeg video formats (the decent ones) were around shortly following quicktime and were used, though quicktime took the main line due to it's promotion (something Mpeg video didn't really have at this point).
And now you know why quicktime is used
