It's more to keep scope small until we feel confident about the experience.
Theoretically, we can scale to other declarative frameworks, but we'd have to implement a parser for each. We're sticking with React/Next for now since that is the majority of use cases with plans of supporting other frameworks when the experience feels good enough.
There's a comment under about Svelte support as well (which is what the initial prototype supported) and I gave a similar answer there.