I think it's the same reason than in many other countries - there are no fast enough types that want to make the switch from 400m. Runners with ~47-47,5s 400m speed, who are naturally stronger in the last 100m of the 400 could do the switch more easily, but often not willing to do it, because the 800 hurts so much, and MAYBE they still believe that by better 400m spesific training they have a chance to go sub 47, which is ok (but hard to do because of "poor" basic speed), but never will know the 800m ability... Yes the 800 hurts but it hurts much less after a decent 800m preparation. HRE pointed out well about Doubell's 10milers, who believes that they lack some mileage. In these days we are so "smart" that won't believe in the methods coming from -60's, even the guys already of 800m won't do it or something similar, lacking both the speed and the endurance. I'm not saying that you should do the same that they did back then, but ADD SOMETHING SIMILAR for better endurance. Not so surprising if they under-perform at the 800. The increased endurance work in addition to speed and other more spesific training they already do won't make them too slow for the 800m, but it would help greatly in the second lap. If one can do a 47-47.5 @ 400, with better endurance they maybe could reach at least a 5-5,5sec difference per lap (400/800m pace per lap) and go 1.44-1.45.
Same question could be done with milers who do ok at the 1500, but lack the endurance at 3-5k needed for better results in the mile/1500, could go down and add basic speed-anaer.capacity training + strength work and maybe go below 1.45-1.44.