The two programs, to create diesel-powered versions of the 172 Skyhawk and 182 Skylane, have been plagued by technical and vendor challenges, most notably the insolvency of the original engine manufacturer, Thielert, several years ago
There are several key advantages to buying a new diesel Skyhawk, most notably that you're buying a new airplane and not one that's merely "like new."
You also get the integrated G1000 avionics with all of the engine management tools built in, an advantage aftermarket converters are unable to offer at this time.
Buyers also get all of the latest safety improvements that Cessna has made to the Skyhawk since it restarted its production in the early 1990s after a 10-year hiatus.
With so many airplanes type-certified as Cessna Skyhawks produced over the years, this seems self-evident.