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It was Supergirl's season premiere Monday and all people can talk about is Superman. A Superman tv show would completely supplant Supergirl's role and what they want to accomplish with her. Remember how SG ended with CBS. It's not like TPTB wanted to renew the show there.
..and the DC TV material is already viewed as a lower class cousins of the DC movies, so their core TV series do not need any other "distractions" by launching a series starring a character that's part of the Mount Rushmore of superheroes, while the other characters have nowhere near that kind of cultural appeal, market value, etc.
Also there's SG's penchant for taking characters, stories, references and attributes from Superman's mythos and applying them to Kara. As well other characters and stories that aren't directly affiliated with Superman or Supergirl (like Arrow does with other DC IPs) in order to make episodes. There really wouldn't be much room for a unique Superman show. I imagine we'd get something akin to Smallville 2.0. Given Hoechlin's big Teen Wolf fan base and it being on the CW.
Agreed.