Karl,
The M/X bikes seem to me to be the best for the model companies because there's a proven track record of builders who love to build each variant of one particular vehicle. Look at the people who concentrate solely on constructing every variation of a BF109, FW190, P-47, P-51, etc. Heck, I see you doing this with the German armor.
Because the M/X racebikes are continuously updated to keep "the edge" on their competitors, every year is another sales opportunity because unlike automobiles which may go unchanged for years, the M/X bike that doesn't offer the "NEW!" feature to capture the heart of the wild eyed rider/racer gets passed over for the company who does create the buzz with the newest "must have" new feature that the series champion claims was the thing that made it all possible.
Another plus I see in these vehicles is the fact that offered in a 1:12 or 1:9 scale allows the old(er) demographic that actually participates in this hobby to work with parts that aren't so difficult to see and handle and the finished product isn't as space consuming on the shelves as the aeroplane.
Then there's the early bikes like the old boardtrack racers, which could be appealing to those who love the WWI era aeroplanes. Anyone who takes a tour of the Barber Motorsports Museum in Leeds, Alabama would see that the variety of subject matter that this genre offers is staggering. Sure, I'm biased for this, but from a mechanical standpoint, they are some of the most fascinating machines the human mind has ever brought forth.
Regards, Roger (PWB)