Microsoft then had a monopoly on desktop OSes at the time, at 95+ (close to 97%, in fact).
Apple has a paltry sub-50% share of mobile OSes.
The "But they have 100% share on their own products" is not an argument, as far as the law is concerned, as monopolies are not judged this way. That's the same reason why you can't legally force McDonalds to also sell Burger King burgers, or GAP to sell Banana Republic clothes...
And even MS monopoly position wasn't the issue (having a monopoly naturally is not illegal). The issue was it used its monopoly position to threaten and bribe OEMs to not work with competitors.