It’s not easy to jump ship (because of the network effect) and possibly not being tech savvy enough to know where to land.
I jumped ship from Reddit this year after reading some posts about alternatives. I tried Tildes too, which I found interesting, but more of an experiment, than a substitute.
King (and others like him) may not know alternatives exist and can work just as easily.
Also, even if King knew of Mastodon, maybe his fans wouldn’t jump ship too even if he had said he’s moving there