That way they can much more easily acquire software. Seriously making an app from scratch is no small task, and often time isn’t worth it for such a small market.
PWA's and the like can stay away, I want software from developers that actually respect their users, software that doesn't waste resources, software that actually looks native.
My point is it’s so small, nobody will focus on it. However, if they can easily port their existing codebase, it will have more apps.
Contrary to popular trends, programmers can know more than a couple of languages and not everything has to be javascript.
I'm sure many hackers would be more excited to make an app with go, rust, haskell, etc than to do js, that just feels like work.
Some additional thoughts:
1) maybe shipping a new OS with hardware, catering to a privacy focused niche is key to succeed this time
2) maybe web technology wasn’t ready to have wide adoption for making cross-platform apps at the time FirefoxOS was launched, but is now
3) maybe PureOS will fail regardless of how apps are built. In my opinion, the best chance of succeeding will be from actually being useful. In order to be useful, it needs software. Unless, of course, you seriously believe that getting hackers excited by not using web technology will be a better solution.