It is for having a dominant market position in mobile phones (90% global market share) and abusing the market position in mobile phone operating systems to favor their other products.

Apple does not have a dominant market position and does not give away iOS for free.

Making Android free is an anti-competitive practice because it inhibits the formation of a competitive market for mobile phone operating systems. Because Google's practice of making Android free inhibits competition they are not allowed to exploit the lack of competition for profit.