That is down to who has the most funding. Javascript itself has even more "support, completeness and maturity" than Scalajs. But obviously that's not the deciding factor.
I think it's mostly down to determination. Some things are hard, but need to be done.
Haskell people just seem to give up more easily - see all the half-working projects. Nobody finishes stuff, the next dev just starts his own new project, and abandons it later.
Scala.js handily beats JavaScript in terms of IDE support and tooling
The Haskell community has achieved great things even though most contributions are done in people's spare time. That is determination.
There is no determination in failing to get anything done and doing NIH for a decade.