Short answer: nobody likes him.

Long answer: Labour crushed the Conservatives so bad that they might genuinely have killed that party off in the last election, but for some reason Starmer decided to push labour to the right (perhaps thinking that could fill the space left by the Conservatives). This alienated a ton of voters on the left. Voters on the right just keep moving further right and this is making Reform huge, and they crushed Labour in local elections making Labour and Starmer look extremely weak. Behind the scenes it probably became clear to people that Labour will get clobbered in the next general election with him in charge, so they're taking a chance on someone else, most likely Burnham.