You are mixing productivity (getting things done), quality (better designed languages) and popularity (large number of projects) in the same sentence, but you generally cannot find causal links among them.

In Haskell a lot of it is due to people thinking they need to deeply understand everything before cranking out code. It's like trying to understand music theory before strumming some guitar strings... actually it's probably something less useful than that.

However, I guess the inability to deeply understand or pontificate about Go is an advantage in itself... forcing your only potential activity to be getting work done.

Also, are you sure that there are more popular Go projects than popular Scala projects?

That may also have to do with the massive push behind it.

that would mean javascript is better than go for GTD ;)