Wanting to abolish the directly elected mayoral position maybe?
Exactly the same happened in Liverpool all three prospective Mayoral candidates were told they couldn't stand because all three wanted to abolish the DEM. I'd describe the three candidates as Centre, Left and Very Left, one black two white
The extgreme left of the party jumped up and down in exactly the same manner, stitch up, local representation (ignoring the obvious elephant in the room as here), racism (the black candidate was the most left wing), sexism (all three candidates were female), one candidate even attempted to sue the Labour Party (and lost, heavily). None of the accusations were true, they eventually elected a black woman from the same ward as the other black candidate who happened to share a name similarity with the outgoing disgraced outgoing Mayor (which wasn't even close to ideal)
If you are a political party, that has concentrations in urban areas why in your right mind would you want to get rid of en elected mayoral position that you are nailed on to win The answer in Liverpool was because it was a knee jerk reaction to the corrupt incumbent, I suspect in Leicester the answer is somewhat differnt and is down to policy differences between the elected mayor and some councillors, There was similar feeligs by a minority of Labour councillors in Liverpool, they've left and formed an independent group and will in all likelihood become an irrelevance over the coming election cycles
The idea that removing elected mayors is somehow more democratic is obviously a question rarely answered by those that oppose them. Starmer and the Labour Party are obviously against removing DEMs so can't have candidates who differ on that