The constant internal battles within the Labour party are tedious.
I imagine the leadership know that if they win the next election, there's a very good chance they'd need to form a coalition or would have a pretty narrow majority (I'd be shocked if they can win a majority along the lines of Blair's 3rd win in 2005). Under that scenario the block of left-wing Labour MPs would wield a lot of power, and be able to make demands in exchange for not inflicting numerous Commons defeats on the (still hyopthetical) Starmer government. On the flipside, had the last election 'somehow' (as increasingly far-fetched as that looked during the campaign) for a Corbyn led administration, the sizeable number of 'centre left' / 'centrist' Labour MPs hostile to him (much larger than the current block of 'left wing' MPs) would have had tremendous power and surely have forced him to water down his programme significantly.
So from their perspective, the fewer MPs / candidates at the next election on the left of the party, the better.