Crystal is for when you want to write C with Ruby-ish syntax. It's been around for awhile, all the way back to when Ruby was required to get VC funding.

In my opinion, it doesn’t excel at anything beyond being pleasant to read. Maybe if Go hadn’t come along, it would have gained more traction and developed a better concurrency runtime. Very few companies are publicly using it.

Plus, since 00s Ruby = Ruby on Rails, so it didn't appeal to ruby (on rails) developers back then.