It's LP rules. The NEC decides whether or not someone can stand as a Labour candidate in a constituency.
Starmer and McSweeny don't want anyone who is likely to challenge for the leadership and who has more socialist beliefs than the LP is currently pursuing.
I think there will be a challenge, anyway, after Labour do really badly in the council elections, But it will probably be Sweeting, who is in hoc to private health companies and who is no more 'left' than the current leadership.
I think this could be the death of the Labour Party.
I think the by-election will be fought out between the Greens and Reform.
Rayner might be a bit of a dark horse in all this...