AFAIK, the switch to RPM happened when Nokia was still in the game, as part of their joint agreement with Intel that transformed Maemo into Meego (which is now Jolla). Intel's own "linux for phones" project was based on RPM; the idea was that Intel would have brought better OS internals, and Nokia would have provided a better UI, playing to the strengths of both projects.

By then the development of N9 was already at an advanced status, so it didn't switch to Intel-provided internals; the N9 is basically the last Maemo device, and everything Jolla did afterwards is the Meego we would have seen if not for Elop's self-sabotage.