Without expressing an opinion on Flutter as such, it's worth remembering that Google is a big enough organisation that even -some- internal apps being built using it is quite a commitment, and it not having spread further internally could easily be a social or political rather than a technical issue.

I don't think it it's a technical issue. On a technical level Flutter is mostly great. But Google has been doing aggressive cost cutting lately and not having Flutter in use in that many apps makes it more vulnerable.