For one thing, Rive is built for runtime and interactivity. Unlike Lottie, which is built on the After Effects format to basically generate vector gifs.

More on why that’s important: https://rive.app/blog/a-new-graphics-format-for-the-interact...

Some comparison numbers between Rive and Lottie: https://rive.app/blog/rive-as-a-lottie-alternative