Recently, I've decided I want my main AU continuity that still needs a name to include a handwave that Mara lives, and it occurred to me that all else being equal, she wouldn't be able to go with Luke and Ben.
The original terms of Luke's exile were made on the assumption that Luke would go alone. He couldn't request information or assistance from Jedi resources. Ben wanting to go with him was a last-minute change of plans that the GA couldn't have found out about in time to stop, but Ben's still technically in the Order. I think the only reason the GA didn't hold that up, later, as a violation of Luke's terms is that Ben doesn't have sufficient rank for Luke to exert meaningful influence on the Order through him.
But another Master? Only if Mara also left the Order would she be able to go, and they both know she probably could do more good by staying on Coruscant and helping with that.
