Because Billy is often treated as DC's other Superman character. If you ask me, Billy could work in the same space as Diana because writers can play up their connections to mythology. Whereas, Billy is almost always shoved aside for Superman. Writers flip-flop on Wonder Woman. And MM is written that he's stronger than Supes, but hardly ever shows it (to my knowledge). In fact, Diana been DC's go to for taking Superman down for most of post-crisis (I usually lump up New52 as post-crisis).
DC Comics other powerhouses just don't work Superman. Because most writers are going to keep Superman as the muscles.