Having built an electron app with linux support: it takes time and effort to get it right, you have to come up with a different updating strategy than windows/mac, and they might not have many users on linux at the moment (low priority).
It's not as simple as "add a flag to build linux." There's a little bit more to it.
Some people like to have things in separate apps (even if they are not native) just so you can open and close it quicker than finding it in your tabs or getting distracted by all the other content open in your browser.
Yes it matters. I could write a WebKit2 wrapper for the webapp in 10 minutes just so I can treat it a like a standalone app and not just another tab.
If it's electron, give me linux.
It's not as simple as "add a flag to build linux." There's a little bit more to it.