AFAIK Harmattan UI was supposed to be an evolution from Fremantle/N900 UI but in the end something that was originally developed outside Nokia was used to simply get the device out in reasonable time. Personally I preferred the N9 UI, it was at the same time efficient, yet elegant and different in every day use (as my days of trying to actually do something productive on mini-size device are by far gone). Perhaps not for extreme multi tasking tough... also, there is strong NIH-attitude in most big corporations, so there may be cultural etc. reasons for not simply copying some UI paradigm to begin with.