Here's one reason that a longer bow is more stable: leverage. It's plain and simple physics. To demonstrate it take a broom handle and put a weight on each end of the handle, hold your hand in the middle and with it out at arms length try to twist/lift one end and then the other. Now do the same thing, using the same weights, on a handle half the length and see what happens. (It takes half the energy to twist the handle/lift the weight). That's why a long riser bow is harder to torque than a short one, the longer one takes more energy to pivot it on the fulcrum point of your grip.

The other reason is mass; because the riser weighs more it also demands more energy to move. (and as an added benefit, absorbs more latent energy from the bow at the shot which makes it quieter.)

The only advantage I see to a short riser bow is that it weighs less which is a huge issue for me lugging it up and down 2000' mountains every day but shouldn't matter to the average hunter that just pulls it 20' up a tree. (I even poured lead into the limb pockets of my target bow.) Even given the weight issue I refuse to own a bow shorter than 36" and it took me a long time to try one that short with my DL at 30". In general someone with a shorter draw can shoot a shorter ATA but that gets into a whole other aspect with 'finger'/nock pinch (not such a big deal with 'D' loops and release as it used to be with fingers).

Art's got a good point about the marketing/materials aspect. Billets of 6061 aluminum and CNC machining/tooling ain't cheap!