The situation is pretty simple. The N9s may make great development devices, but the hardware adaptation that's available for it have a clause about commercial distribution forbidden. This means that a company like Jolla can't do anything - on top of the expectations that 'support' means that a non-Nokia company can't do that. That's the very simple practical reason. The efforts that go into Mer and bits and pieces of Nemo, naturally would arrive on Nemo for N9 too.