I don't know why Cessna stopped the diesel 182. I don't expect that they are interested in elaborating on the decision. My guess would be perhaps a combination of small teething problems, coupled with less than stellar market demand and simply not needed [yet] then. It would have been a more costly airplane, for little perceived added benefit in the main market (the US), where Avgas is easily available. I had an order in with SMA for their next generation diesel for a 182 client. We waited two years, and though the engine was promised, delivery was a problem. My client gave up, and bought a new O-550. I'm not aware that the SMA diesel is marketed to be a "new" engine these days.
I very much liked the Theilert diesel in the DA-42, though it had its challenges...
At present, I'm working an STC program to approve a RED diesel in the DCH-2 Beaver. It's installed and running, and I expect that the company pilot and I will test fly it in September.