A possible explanation

I agree that it seems odd that the C-17's seemed to be victimized before the C-19's. But, we have to look carefully at the rosters of the 30's, and one can see that the Rio Grande tended to phase out motive power that was smaller, and more than often, worn out. We see that the C-17's were retired by October 1938, and that 5 of the C-19s went around the same period.
Either the C-17's had a lot of bad luck, or they were just plain worse off than the others. Considering the way that the Gould's controlled the line, it's a wonder that the narrow gauge engines survived the scrappings of the 30's so well.