#1. Isn't ready: King Corbin

King Corbin hasn't had a whole lot of momentum, yet he's been constantly involved in storylines with Superstars for the last four years. It appears as though he was a Superstar who Vince McMahon saw as a World Champion
However, he was one of the few Superstars who failed to cash in the Money in the Bank briefcase, losing to Jinder Mahal in 2017 after John Cena cost him. He has been in a feud with Elias, one where he lost at WrestleMania 36.
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Elias hasn't exactly been a top Superstar and similar to Corbin, he's lost a bit of momentum along the way. Neither has set the world on fire over the last year, though Corbin's most notable moment was his King of the Ring win last year.
He's feuded with Braun Strowman back when he was the Constable of Monday Night RAW and as expected, it wasn't a feud that grabbed a lot of attention. While WWE might want Corbin to be a legitimate opponent for Braun Strowman (even if its a filler match), he isn't ready for that spot yet.