Inertia, accidents of history. Relational databases had obviously-useful properties at critical points in the history of mass commercial adoption of computing. As a practical, real-world matter, "object-oriented programming" barely existed during the rise of relational in the 70s, making the practical problems caused by the impedance mismatch either non-obvious or seemingly unimportant.
As a result, huge investments were made in advancing relational databases, and other forms have languished. Today, relational maintains the substantial advantages of maturity and installed base. Performance, reliability, general polish, ease of access to support/tutorials/other literature, and general status as "the way things are done".
As a result, huge investments were made in advancing relational databases, and other forms have languished. Today, relational maintains the substantial advantages of maturity and installed base. Performance, reliability, general polish, ease of access to support/tutorials/other literature, and general status as "the way things are done".