I don’t think it’s a regulation as such, more a standard from all banks not to reveal detailed information on these kind of things.
They don’t want to give any detail on why accounts wont be accepted so that people don’t learn the thresholds and game the system.
If they do, then Monzo will be held accountable when stuff does go wrong for not having adequate checks / processes. Which is likely a regulatory requirement.
Even something like “sorry you can’t be accepted because X”
They would then tell someone else “sorry you can’t be accepted because Y”
They would then tell someone else “sorry you can’t be accepted because Z”
And before you know it you have all the pieces to the puzzle.