Because to parse it legally you'd have to store the entire file in memory. That's not actually streaming. The CSJ format puts the object schema and order up front and just applies it to each line afterwards. Stream away...
This is likely content coming down from an HTTP API so likely already gzipped. Since you can gzip both CSJ and JSON I think that's not really relevant.
With this proposed format you only need to parse the object definition once and you also get the ordering fixed. From then on you need only validate each field in each line as being legal and you can build your object quite fast.
This format saves probably half the network bandwidth, half or more in memory requirements, and more than half the cpu complexity in parsing and getting your final objects instantiated. That's pretty good.
Remember the goal isn't to replace JSON. It's to replace CSV. JSON and CSV have different goals. This is simply a JSON structure built to meet the same goals as CSV. It seems to do that quite nicely and can use existing JSON parsers internally to do it. Pretty nifty.
"Vulcan will be partially reusable (eventually)"
Haha what a joke. The whole point of reusability is to be able to land the rocket, refuel it, and send it back up the next or even same day!
"ULA says it's going to try to capture only the most valuable parts of a rocket"
So... how about when we fly NY -> LA in our 777 we only bother to land the passengers in the remaining "valuable" parts of the airplane? See the point ULA? We're trying to make spaceflight just like commercial airline traffic. That's the ONLY way to make it commercially viable. Your cost-plus methods of building expensive as possible single use bottle rockets are over man!!
This is a pretty amazing resource if you are a webservices speed freak and need to find the lowest latency possible for users in a particular geographic location. I haven't seen so many services rated in a single place before and never seen one that actually updates the ratings so frequently.