Three of the K28s were based out of Salida and Gunnison: 472, 474, and 479 and the 72 and 79 were used in freight service after the Shavano was discontinued. I am just guessing, but I suspect they were chosen because they were not needed up there. The two primary bridges on the Crested Butte branch were strengthened in 41 and 42 and the K37s were then permitted on that branch, so they could give up the 474.
I have seen no documentation on why the 473, 476, and 478 were chosen to remain. The Army wanted all ten K28s and the D&RGW had to enlist the aid of Colorado congressmen to keep the three for the San Juan. If the Army had gotten their way, there would be no K28s today.
I would guess that the three probably had recently been shopped and were probably in better shape than the others. They would not have wanted to keep 474 as it had no steam or air lines...shame to me as it is my favorite engine.
Jerry Day