It's a mix of a number of factors not just party gate.

London for instance has previous experience of him. It seems that they have lost some dead certs this time.
Pundits feel that the % vote spread in locals can indicate the likely results of general elections. On that basis the Tories will loose it and Labour would need to form a coalition. That is an improvement for them. Get another 5% and they might win.
My view as with May is that Boris will be a scapegoat at some point. It's just a question of when. He's helped achieve what his cabinet and backers wants. Some other Tory are referring to themselves as something else. The rot is beginning to set in. The cabinet related ones are still around such as the chairman of the party etc. Bizarre thing is May might make a suitable replacement. Sunak ??? Could he ever agree with who ever gets his position or try and do 2 jobs at once? Brown was probably a mistake.
I think I heard that the LD are toying with the idea of offering another route back into the EU. Then there is the Greens - more support now. Maybe the next government will turn out to be another coalition. Mass media allows all sorts to get the oars in. IMHO this means that politics will never ever get back to how it used to be.
Take a major news item now, beergate. Nice name popped up quickly and all totally meaningless until the police make a decision. That wont stop it from influencing people.