[seanc]

I used Datomic for a few years at a job.

In addition to the cost and proprietary nature, I'd say there are two reasons;

1) New and different things come with risk, and many folks are risk averse, especially in groups

2) More concretely, it is very easy to write slow queries in Datomic, and it can be a struggle to diagnose why the order of your datalog statements matters so much