There are two things driving diesels in Europe:
1) Tax concessions (and these can do do disappear, and don't exist in the USA)
2) Reliance on avgas (which is not an issue for those suitably based who don't fly far, and is not an issue in the USA)
So, in Europe, take out the reliance on avgas and what have you got? A bet on future tax policy...
Diesels do have a bit better SFC, due to their higher compression ratio, but this brings technological problems with the severe torque pulses going into the prop. I am sure this will be solved eventually, but the jury is going to be out on long term diesel reliability for a long time.
I agree but the problem is the aviation market isn't big enough to spread the development costs of new engines and make a product that everyone can afford. Lycoming and Continental had the luxury of living through the hey days of GA aircraft production in order to develop and refine their Avgas engines. Those days have gone, there were more aircraft sold in a few weeks then than there is in a whole year now.
I think that is very well put, but I also think that the current diesel retrofit options are priced right at the very top of what is perceived anybody anywhere might possibly pay, with the "anybody anywhere" being the busy flying school business model. This doesn't exactly help wide adoption.