As far as I'm aware, there's nothing preventing a PluralKit equivalent from being made for other platforms. In fact, a quick search turned up a WIP Matrix port on github.

So no, I don't think this is true. Lack of PluralKit isn't what's preventing people from switching en masse. It's the opposite—lack of people switching means there's a lack of demand for a PluralKit port in the first place, so even though there is a port people don't know it exists and thus it doesn't get as much dev attention.

It comes down to network effects, ultimately, and just plain inertia. If you're already on Discord, and all your friends are on Discord, it's hard to convince you to switch. And being more familiar with the Discord bot ecosystem (like PluralKit) is just one more thing that adds to the inertia.