It started as a bespoke structured data and Web architecture, which did somewhat complex things that would've been a huge headache to build and maintain in the dominant languages of the time, but were viable for one person to figure out and implement in Scheme and PostgreSQL.
That bespoke architecture and implementation language lent itself to a lot of rapid domain-specific functionality, as well as doable architecture changes over time.
There's actually a bespoke object metamodel mapping atop the RDBMS, and it permits customers who want to use SQL direct to the database to do so.
And, IMHO, PostgreSQL is the default choice for most storage backend purposes, unless one has figured out a good reason something else is better. :)
There were also multiple kinds of large blob storage needs, and changing architecture needs over time (e.g., multi-tenant, changing authn implications of things like browser plugins, scalability, move to cloud), so systems programming skills and controlling your framework comes in handy, for not getting stuck or blocking on vendors, but just solving the problem.
That bespoke architecture and implementation language lent itself to a lot of rapid domain-specific functionality, as well as doable architecture changes over time.