Very similar to asking why you should use React when Angular came first. A-Frame and React VR represent different styles, and they both excel at different scenarios. If you're building a relatively static scene, or really like the idea of having a small amount of HTML with super powers, then run with A-Frame. I think it's a fantastic product.
If you know React, find React's declarative style of programming desirable, or have a more complex, stateful application, then React VR is your best bet. It's true that the A-Frame community is larger than the React VR community, and there's more code out there for it, but React VR integrates with the entire history of React tooling and libraries – React VR doesn't need a specific data management or navigation library, because it's just React. Our goal is to take all web developers who know React and enable them to become VR developers as well.