We are always trying to reduce the learning curve of FQL, but we've gotten a lot of good feedback from those who do take the plunge. That being said, I'd love to provide SQL support eventually, but there were a few reasons why we don't support it as the primary query language for FaunaDB:

Our core use-case is OLTP, and we wanted to address the shortcomings of SQL in this context, such as query performance being highly unpredictable in the presence or not of indexes, whims of the optimizer, etc.

SQL is just not great as an application-level interface: Tables are not a natural fit for many data models (the classic impedance mismatch problem), programmatic composition is difficult. We want to obviate the necessity of an ORM library on top Fauna.

SQL the language is not great for writing complex transactions in, and session transactions require a lot of back and forth between the client and database. We wanted to make it easy to write queries which can encode as much business logic as possible, hence FQL's semantics are a lot closer to a regular programming language.