Why can't the web+JS be that platform?
But even if we ask the more general question, why "web+js can't be the great cross platform toolkit", the answer IMHO is:
Web+JS are and will always be (due to the nature of the platform) inherently slower than optimal (even if "fast enough" for simple apps), use more memory, use more CPU, don't use the native facilities and so are foreign to every OS look and feel (from themes to usability features), and so on. And of course they're "bring your own theme" affairs, so no standard users can get used to, everybody does their own BS UI in SPAs and web-engine based apps (like Spotify and Atom).
Add the amateur hour development practices that they enable (where a company that would previously hire actual GUI programmers can now hire some web devs and let them loose to create its app) and the picture is not nice...
What's worse is that actual native toolkits are rotting, while at the same time we never managed to produce a single, great native cross platform toolkit -- even less so a great cross platform one.
Not because it's impossible, but because no major player cares (they have their own platforms), and OSS attempts like Wx and GTK+ don't have the manpower.