Didn't read through the entire thread but anyone that has experienced startups will have seen that the stack is decided by the tech co-founder/ first engineer. This is because they are often still the only engineer. And they will decide on a language they already know - a language that is not popular among startups is a language not popular among engineers that join early stage startups. I could imagine that many enterprise developers using Java or dotnet have relatively comfortable jobs and tend to not relocate.

There is then the point where a startup may need to "step up" to a more large-scale language, for example when Twitter started incorporating Scala. But succeeding enough to reach this point is a small subset of startups, many probably not even called startup anymore, so any language at this stage may not "feel popular" when limiting to startups.