I am a lesbian and have been going to various UK pride marches/events since my teens (so 20 years now).
Pride was already a growing corporate 'thing' long before I came along, esp the big city ones, but that has absolutely mushroomed. Alongside that, the change in public attitude towards LGB people over those 20 years has sent both the attendance numbers and the commercial appeal for sponsors etc skyrocketing. Straight people (genuinely straight, not spicies etc which is a slightly different issue) see it as a nice cultural day out to join in with. Which is both a mark of progress (people positively want to hang out with the gays! People aren't offended if they're mistaken for gays any more!) and also, IMO, a bit of a nuisance. Obviously you can't police Pride (and I wouldn't want to), and not everyone will agree with me, but I feel that it's for us, not them. Because even now I think for a lot of LGB people, coming out is STILL a really big deal, and Pride as a concept is relevant. There is still plenty of gay shame to be exorcised, sadly.
So in that sense, I think Pride attendance numbers are due a bit of a correction.
Major companies deciding this isn't the thing for their money - perhaps also due a correction. Perhaps they see toxic issues emerging wrt the TQ+ part of Pride. Perhaps a party atmosphere doesn't chime with their view of the consumer market in a world which is in debt, at war, in financial and climate crisis. Perhaps they're on the lookout for the next big thing. Maybe a bit of all of these things.
I've swerved Pride in recent years. I'm really done with the forced teaming and being expected to happily align with all this #LwiththeT whatever. The fact that I believe TWANW is not the focal part of my political beliefs, I don't actually want to dramatically part company with all LGBT politics and campaigns over this, but there doesn't seem to be any neutral space left. On top of all the corporate sponsors (Barclays Bank at the front of London Pride ffs 🤮) and the crowds of straight people, this obsession with performing trans inclusion has felt like the last straw.