There is no such thing as "the iPhone market" for anti-trust purposes -- there is the mobile phone market. And competition in that market is inarguably healthy. Customers have plenty of choices that aren't iPhones, and largely they seem fine with them. You might as well argue that Chevrolet has a monopoly on the Corvette market.
Microsoft got into trouble because they were found to have a monopoly on the personal computer operating system market and then they deployed that monopoly to shut out competition in a separate market -- web browsers.
No one said that Microsoft had a monopoly on the "Windows market" because that's a tautology.
The fact that they were successful enough to obtain a monopoly in the PC OS market wasn't itself illegal. Their usage of their position in the OS market to gain an advantage in the web browser market is what was found to be illegal.