You have to remember that the whole point of the 900 was to build a lower-cost version of the 1000. So, obviously, the dash 5 engines, which made going too much above FL310 a moot point anyway. And I think maybe as a result of that, it had a lesser oxygen system or something, related to certification-- but I could be wrong. I've never flown a 900.
They were looking for places to save money on it, in order to bring the price down.