From this interview with Rich Hickey:
Q: So why don’t you like relational databases, why did you create a new system?
A: One of the objectives of a lot of new systems is to decentralize things in order to get greater scaling and have more flexibility. So there is actually a lot to like about traditional databases. I think relational algebra is a powerful tool , and I think transactions are powerful, so we want to retain transactions, we retain a relational-like model which we expose through a query language called Datalog, which has the same power as relational algebra. So we value those things quite a bit, but you want to move away from having one server or a cluster of servers do all the work, because it’s very difficult to scale that, and in an attempt to scale that, people are giving up some capabilities.
Original title and link: Datomic: Datalog, Databases, Persistent Data Structures With Rich Hickey ( ©myNoSQL)