That distinction is a historical accident. You can page a program heap to disk and you can run a relational database entirely in-memory (this is one of the common use cases for sqlite). True, databases and programming languages are usually optimised for different use cases but that says more about the particular implementation and the way we think about using them then it does about the data models.

The question in both cases is how should we model, query and update our data. The original tradeoff was that hierarchical databases and network databases were faster but relational databases were more flexible and maintainable (due to the separation of logical and physical data representation). Better implementations (eg http://en.wikipedia.org/wiki/IBM_System_R) narrowed the divide and made relational databases more attractive. Systems like Bloom (http://boom.cs.berkeley.edu/) and LogicBlox (http://www.infoq.com/presentations/wavefront) show that the same tipping point could be reached for programming languages.