In WebGL frameworks, Playcanvas is like Windows where Three is Linux. You can do the same things in both, it's a bit easier to start with in Playcanvas, but the cost and freedom of Three attracts lots of developers.
But the analogy also falls apart in certain significant ways, e.g. PlayCanvas being open source and not actually tied to their servers or editor or anything (but don't get me wrong, it's not as easy to add into a standard project structure as `npm i three` either).
> but the cost and freedom of Three attracts lots of developers
There's definitely some truth to that, but I'd bet it's a very small fraction of three.js users who are even aware of PlayCanvas' existence, and that that lack of knowledge probably affects adoption ratios significantly.