The issue is largely related to Apple's dominance in the mobile and tablet OS markets. Apple (iOS) and Google (Android) control most of those markets. According to Statcounter, for example, iOS has around 61% share and Android has around 38% share of the mobile OS market in the U.S. They're the only two major players. Companies with notable dominance, power, control, etc. of a market can face antitrust scrutiny.
You don't see that kind of dominance with GM, Ford, Toyota, Honda, Hyundai, etc. in the auto market.