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.
I’m still waiting for a high-end second generation version. From the various forums, there are/have been some minor bugs and I’m somewhat annoyed that they went for RPM instead of sticking to dpkg from the N9, but otherwise it seems a sensible approach to a mobile phone without the usual issues with Google/Apple.