I've never been much of a RC fan, but this pretty much sums up how you could solve every issue in this entire thread.
Dirt Bikes are relatively cheap because of the huge amount of carry over year-to-year. Your amortization of engineering and development costs, and of molds, over a number of units isn't about the number of item X you make in a year, it's about how many you make over 5-10+ years.
You also can have a thriving aftermarket (accessories / tuning / customization) because those companies know they are investing in products that will have long term compatibility (imagine how many sets of levers Straitline had for Juicy's and Shimano XT/Saint/Deore brakes when all of a sudden BAM! Elixir and Servo-Wave!
For all the flack he gets (or had gotten) the dude has some very good viewpoints on the industry. He's been around forever and has seen everything I'm sure.
In regards to the motorcycle industry, you are correct, as most brands tend to operate on 5 year rebuild plans. That is, complete ground up makeovers on dirtbikes, with year to year smaller changes. Some brands in mountain biking seem to completely revamp their lines every year. New carbon molds every 2-3 years, that kind of thing.
It also has to do with the number of units sold. Its also quite a bit different in that you have the big 5 (KTM, Honda, Yamaha, Kawasaki, Suzuki) each selling a handful of off road models. I wonder how many KTM 450sx's were sold in comparison to how many Giant Reigns were sold? I'm sure the numbers are out there. Let's say KTM sold 30,000 450sx's vs Giant sold 5000 of that one Giant Reign model. Get my point? There are a million mountain bike brands selling a bunch of models, with variances within the models. Some mtb brands offer four builds within the same model! That's just crazy. If they would offer two, they might not lose any sales, they could probably sell the same dollar amount spread over half the number of sku's.
Then the California equation comes into play, where there may be as many legal motocross tracks as there are good legal mountain biking areas. If bikes are similarly priced, you know the kid who lives near a track in suburbia with no trails will pick the moto all day long. Decent mountain biking areas seem to be more isolated and fragmented in comparison. If you look at the number of units sold, California largely dictates the motocross industry Worldwide.
I could be wrong on all these points, as it is only my semi educated viewpoint. In regards to the market though, the two "sports" couldn't be any more different.