Maybe antitrust distracted them but not so much to miss huge mobile OS opportunity. They were destined to make mobile OS because they had so much knowledge about operating systems and they had so much resources to do it. I also read somewhere that Gates wasn't excited about mobile phones and thought that PCs will rule the world forever. On the other hand he was excited about Xbox that's why they invested much more effort and resources into gaming hardware than in mobile phone.

Gates even admitted he messed it up and lost $400bn with mobile operating system. https://youtu.be/W5g4sPi1wd4?t=709