[crabbone]

I'm not the author, and I don't know how author would answer this question, but some things are really on the surface:

1. Modularity. In Datalog it's very easy to name a common (sub-)query and reuse it in multiple queries. You can have stored procedures / functions in SQL, but the syntax is very different implementation to implementation and users are typically afraid of using the feature (the SQL/PSM does not go far enough to define what users would normally want). Also, SQL/PSM is an imperative extension of otherwise declarative language. The two just don't work well together.

2. Structures other than primitives and tables (you get lists, vectors, hash-maps and sets, but you can also make your own). A lot of practical SQL extensions also offer some of these structures, but they aren't in the standard.

3. Operations on primitive types fall into the same category / work in the same way as operations on complex types. I.e. select / update / insert / delete in SQL only apply to tables, but strings or numbers don't work in the same way. It's more uniform in Datalog (not 100%, but still better).