why can't he just deploy BIP-300 and have his drivechains since apparently nobody needs to be convinced of anything to activate a soft fork?

I think his point is that only miners need to run it. "Regular" nodes can go push daisies because they don't choose what goes into blocks, miners do. Note: I don't personally think regular nodes can "go push daisies," I am just trying to use colorful language to represent what I understand Paul's opinion to be.

Also, I think this "miners-only" point is related to the fact that when soft forks are up for activation, regular nodes aren't asked to do any signalling -- only miners are. So, from Paul's perspective (as I understand it), he only needs to convince miners to run his software, no one else. Unfortunately for him, it doesn't seem like miners want to run his software either.

I suspect miners like having the regular, constant stream of professional updates they get by running Bitcoin Core. If they instead ran Paul's Mainchain software (which is his fork of Bitcoin Core with bip300), they would have to rely on him and his small team to upstream the upgrades made by Core. And that sounds dangerous and unwise.

So they just stick with Core, and thus can't be bothered running his bip300 software. I believe Paul has stated that he has personally discussed bip300 with a bunch of miners, and almost all of the ones he has talked to about it are on board with the idea; except for the part about running something other than Bitcoin Core.