Primarily it's not the language that makes people more or less productive, though it does have some influence. It's mostly the frameworks in those languages. And traditionally the most modern / full-featured web frameworks haven't been in systems languages. The major counterexample at the moment (while still obviously not a systems language) is that modern JS VMs are actually really fast, so while I don't love JS, it does hit that sweet spot at the moment of performance and mature frameworks.
Also, I've never worked in Rust, but am mostly a systems programmer, and while I understand that Rust is supposed to be easier than C or C++, I'm skeptical that it's as easy to work with as higher level languages, or that you could throw most web developers into Rust without some serious additional learning.