More to do with Homologation (regulations) and you'll find that apart from the CLK that there's a hole between the GT cars and the saloon type DTM cars.
Basically the coupe's don't have a series to race in so they don't make them.
This is the closest I have seen. Dont know if they were raced anywhere or were just high speed prototypes. BETTER PIC under mercedes on the mosselman site. http://www.mosselmanturbo.com/index2.htm