Styling on mobile context and desktop context are two different beasts. This is why I decided against fully replicating the CSS system with Jasonette because most of it is overkill.
Plus we need to remember a lot of web technology is based on trial and error. In 2017 we have a pretty solid CSS styling system but if you look deeper, it's littered with unused and deprecated attributes because they don't make sense anymore (or the committee probably thought it was a good idea at the time but realized it wasn't so decided to retire them)
Anyway, Jasonette is aggressively pragmatic and it follows the mobile native layout system. It does its best to make the syntax look familiar so developers feel like they're dealing with a JSON version of HTML but they are completely different things.
Coming back to the topic of CSS grid, it may sound powerful (and it is, if you're building a website), but the whole thinking comes from desktop-oriented mindset. To be more precise, neither iOS nor Android have a native layout system that's equivalent to CSS grid. What they have instead are
UIStackView (iOS) and
LinearLayout (Android) which are very similar to how Flexbox works on the web. This is why Jasonette layout system is implemented the way it is. Instead of trying to replicate the web, it comes from mobile-first mindset and tries to make it as familiar as possible to users.
And it's working pretty nice, especially in a mobile context where all the complexities are gone. So I don't think we'll be switching to a new CSS grid like system anytime soon.