It's not necessarily the 'decline' of 26" wheels, but the tire MFG's have to make tires in 26, 650b & 29' sizes, if the make a 1000 tires in 1 size, they are not now going to make 3000 tires for 3 sizes, they are still only selling 1000 tires, so the split it up 3 ways. If you are a bike store you also can't stock every tire in every size. All this lead to a more limited selection.