Why would they? They’re expecting to be the dominant party of power with the Tories having shat the bed for a generation (see also: 1997), and damned if they’re going to share any of it with any lefty troublemakers.
by abolishing STV and putting it back to first past the post, they hoped the outer boroughs' vote would be enough to swing it in the Tories' favour. Fortunately, Susan Hall was so bad a candidate even large chunks of outer London went "nah, mate, not having that."