Why no new diesel radials?

The "Lycoming-Detroit" opposed motor is actaually the Italian V-M of
V-M Motori, a onetime company owned by AlfaRomeo. This is the 2nd time
that americans have attempted to market this line in the US. It is
modular and very economical. Back in the 70's the attempt to
distribute failed due to lack of experience and a serious investment in
the future. When FIAT "bought" AlfaRomeo, they did not want the sister
companies of Alfa, so they were sold off, along with AlfaAvio. V-M
automotive motors are very successful due to their low noise levels and
extremely low untreated exhaust emissions.

Further to the radial comments, Dont forget that the Packard radial
Diesel was built in Detroit and was not popular with airframe designers.