The problem IMHO is that there aren't a lot of problems that are particularly suited to STM.

I can't think of many instances where I would use STM over CAS (e.g. an atom in Clojure); and the times I have needed something akin to STM (e.g. building an incremental dataflow engine with transactions) I wanted more control over the runtime behavior than Clojure's STM provided.

Plenty of business systems need transactional guarantees and either durability or distribution (or both), which makes an external store like Postgres or Redis a ton more attractive than STM within a single process.