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.
Scala.js was largely done by a single student on top of his other university duties.
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
That's nonsense. I think it's great that outside contributions (like allowing the use of the Scala's CI infrastructure) are valued, but the actual commits show a clear picture.
There is no determination in failing to get anything done and doing NIH for a decade.
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