Some problems are hard, common and already solved. Sometimes solving these problems yourself adds no unique value to your product. In that case, you should just use a well-reviewed external ecosystem package.

A large project will run into many of these problems. It is a waste to re-invent the wheel because a cool new framework has too few pre-made wheels.

Lack of libraries, as sibling says. Any libraries that exist are not generally well supported relative to similar libs for popular frameworks. Much smaller community, much less ease at finding others who may have had similar issues. Fewer eyes on, fewer bugs picked up. Fewer contributors, slower fixes/updates. Docs there are tend to atrophy faster.

Developed with Hyperapp on big prod apps about three years; was used to inject small amounts of interactive functionality into existing apps (.Net with jQuery powered frontends). Sorta worked, nice, simple idea, but docs almost nonexistent at the time (to be fair there's very little to it), was in process of being dumped for React just as I was leaving (to work on apps with Elm frontends, which Hyperapp takes a lot from, and suffers from similar issues, heh)

I interpret this as you need to do everything yourself rather than using off-the-shelf libraries.