Since it doesn't have as much interest from FAANG people treat it like its dead meat. Rails powers a ton of smaller apps (and some very large ones) but everyone thinks their idea is the going to be the second coming and therefore it needs all the insane optimizations that power Google etc.

The money is in working for a FAANG. So that's what Devs trains for.

There's not much money in working for a small saas unless you're the founder.