I guess you could also ask why there aren't any fuel coupes either? It's a bit of a history thing really. Back to why there aren't any fuel coupes, the gas coupe class exists because guys would bring out their engine swapped whatevers back in the old days and a lot of the time, these cars were multi-purpose vehicles. Some were still street driven, some were drag raced and then raced on the dirt. You didn't run your street car on fuel - you ran gas. Same thing with mod sports. Mod sports was gas only for a very long time because mod sports were still largely street based cars that again, ran on gas, not fuel. The same thing goes for pickup trucks. A lot of times the trucks that were raced were push trucks! Again, same story - nobody ran their push truck on fuel. So in a way it's about keeping things like they were and preserving a little bit of history and how things were done. Oh another gas only example - street roadster. Why only gas? Because you don't run fuel on the street!
Maybe it's old thinking but I think it makes sense. The push to get fuel into mod sports made sense because MS cars evolved from basic engine swapped Corvette into defacto streamliners. And we still don't need any more classes.