One thing I want to add to the other (so far) good responses: They also seem to build Jai for a means to an end, which is: they are actively developing a game engine with it (to be used for more than one project) and a game, which is already in advanced stages.

If you consider a small team working on this, developing the language seriously, earnestly, but as a means to an end on the side, I can totally see why they think it may be the best approach to develop the language fully internally. It's an iterative develop-as-you-go approach, you're writing a highly specific opinionated tool for your niche.

So maybe it's best to simply wait until engine + game are done, and they can (depending on the game's success) really devote focus and time on polishing language and compiler up, stabilizing a version 1.0 if you will, and "package" it in an appropriate manner.

Plus: they don't seem to be in the "promote a language for the language's sake" game; it doesn't seem to be about finding the perfect release date, with shiny mascot + discord server + full fledged stdlib + full documentation from day one, to then "hire" redditors and youtubers to spread the word and have an armada of newbie programmers use it to write games... they seem to much rather see it as creating a professional tool aimed at professional programmers, particularly in the domain of high performance compiled languages, particularly for games. People they are targeting will evaluate the language thoroughly when it's out, whether that's in 2019, 2025 or 2028. And whether they are top 10 in some popularity contest or not, I just don't think they're playing by such metrics. The right people will check it out once it's out, I'm sure. And whether such a language will be used or not, will probably, hopefully even, not depend on finding the most hyped point in time to release it.