Part of the reason is that it's been a commercial database until recently. I also think it hasn't helped that it's closely associated with Clojure (for natural reasons), which might make Java developers turn away from it.
Yeah, the cost is the biggest reason probably. There's probably a lot of in-house developed Datomic clones out there (I know one, that was developed even before Datomic was a thing).
It's a shame, though. Datomic is a marvel.