We can blame CARB for the lack of diesels here. Tier II Bin 5 emission regs are tough to meet compared to the current Euro 5 standard. Like you said however, once those vehicles are required to meet Euro 6, it should make meeting the current North American standards extremely easy.
Mazda would also be the last company I'd be buying a diesel from, you should read up on the debacle that CX-7 owners had to deal with in Australia with their new engines, lol. We were supposed to get a Mazda 6 with a diesel until all that crap happened. What I'd like to see here is the Honda 2.2 i-CDTi.