I've been in the situation a couple of times where we had full control over choosing the frontend framework. Every time we chose React, the main argument was hirability and the fact that the engineers in question were most proficient in React. That is, inertia.

Secondly, when someone new asks me what web framework to learn, I tell them React. And the main reason, every time, is that this is more likely to get them a job, i.e., inertia.

We got here somehow. Clearly, React didn't get here purely on momentum, and must have done something less poorly than the other frameworks. But I think it's hard to deny that inertia plays a big role in its current popularity.

That doesn’t mean they like it now, and more importantly it doesn’t mean it was actually the best choice at the time.

I used to be on a team that used Mongo for a relational database because Mongo was trendy at one time. So all joins had to be done in the application layer.

Most decision-makers make decisions based on what they already know. Most of my colleagues only know React, because they've only been exposed to React. Why do you think Java dominates the market still? React solved a huge problem when it was new, and it still solves that problem fine (change detection - the reactivity model). There are better alternatives available now (Vue - signals), but the difference isn't big enough to create a new monoculture. Sure there are peripheral concerns like how mature the ecosystem is or the particular ergonomics of a framework, but these are mostly fluff.