Datomic requires a storage backend (e.g., Cassandra, DynamoDB, MySQL, etc.), transactors, peers, and a separate transaction log... also, it appears the few real users of Datomic are primarily running it in AWS. Trying to run it elsewhere likely introduces more complications and uncertainties. Good luck getting support, too—you might have to wait for Rich Hickey to finish his hammock nap (ba-dum-tss)
While Datomic's immutability, time-travel queries, and Datalog query language are conceptually cool, they seem like niche features. My guess is that in 99% of the cases SQL can do the job just fine. Also, Rich Hickey may not have braids, but those curls come pretty close.
Without deep pockets for high-RAM servers, many tend to steer clear of Java. If Datomic were packaged as a single binary with fewer dependencies (and no JVM requirement), it could attract more users. As it stands, it's a complex setup with limited benefits, primarily suited for specific use cases, I think (financial auditing / historical data analysis?).
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COLOPHON
(from the Greek word κολοφών [kolophōn], meaning "summit" or "finishing touch")
I don’t have real-world experience with Datomic; I played around with Datascript and ultimately steered away from Datomic after perceiving a low ROI given its complexity.
I hope my fellow fans of Clojure and the venerable 'Simple Made Easy' talk can forgive this little attempt at humor at Rich Hickey's expense! :-p