Because there were hundreds of Windows machines whereas there was only a few variations of Macintosh, all controlled by the same company and priced at a premium.
By this formula, one ought to have been bullish about PlaysForSure--a platform with over a dozen music stores and compatible player devices from almost two dozen vendors. But we saw what happened there: the platform was beaten by a single store with a few variations of player all controlled by the same company and priced at a premium.
Neither example is strongly predictive, but I think this one is a bit more relevant, for reasons which should be obvious.