iOS users are Apple consumers. Among that group there are far more people who: a) are interested in associating themselves with tech-fashion trends and companies, b) have the disposable income to spend (either directly or indirectly via advertising-driven sales), and c) have shown that they are willing to part with said income fairly readily.

I don't particularly care for Apple's products. I find them to be at best of equivalent quality to other items on the market and the aesthetic doesn't particularly appeal to me. But I recognize the value in targeting and marketing to that audience first.