[arohner]

In my experience, STM isn't used because it's very rare to have situations where you need STM but don't need durability. In most production situations, you need to record state changes in a DB or an event log. In that case, the source of truth is now the DB, and your in-memory values are immutable snapshots.

The other reason is that Clojure code is usually well-factored, and most clojurians have listened to Rich and simplified their programs. Not many problems require STM transactions across multiple refs. If you don't have multiple refs changing transactionally, you can use atoms instead for a performance benefit.