I think a big problem is that Flutter's approach is very labor intensive. It takes over responsibility for the UI, which is tough when you want to be cross-platform. You need to keep up with the native UI across all the platforms. (Or, of course, you don't, in which case there are gaps, bugs, inconsistency, jank, etc.) Beyond the UI, it needs to offer integration with platform services and conventions, which is anoth…
And for the latter, the result is something that mostly does what web-based solutions do (with well-understood downsides), only with virtually no ecosystem in comparison.
I do think there are opportunities for cross-platform native development solutions like https://skip.tools/, which offer an alternative to React Native.