The answer’s going to be the same for all models up to the full car reveal and after release. As johniwanna said during Tuesday’s live stream, maybe it’s a licensing issue. Maybe even the FH2 and FM5 cars need to be tweaked for sound or graphics in rain. Maybe given limited time there’s a higher priority for other car models. Maybe the car model in question doesn’t fit the game’s creative vision. Maybe it will be in a future DLC pack.
Until T10 does something unusual by addressing the reasons why a specific model was delayed or left out, post your request to the Forza 6 Car Wish List thread stickied at the top of the forum.