Mostly because it's the most developed and modern one that, at least I, know of. There are others, but they tend to be more niche/specialized, less powerful, or older. Godot comes closest to unity in terms of useability.

But of course, do look for others, if any serve you better.

It's open source and a lot of indie devs like it ideologically. It's flawed, but easy to use and easy to port. Lots of resources on it. Kinda like Linux but for gaming if that explains anything