Most monopolies are only sued (or considered in anti-trust) when they are in a dominant position. In this case, there are two dominant phone platforms... Apple's iOS (iStore only), and Android (multiple phone vendors, Play Store and options to load whatever software a user wants).

In the Apple case, they've taken whatever legal means possible to stop side-loading and alternative stores, dropped apps that try to sell via other paths. They have a vertical stranglehold on the platform, and are in a dominant position.