The original idea was to save the entire Antonito to Durango line. When it became clear that that was unreasonable, they had to choose some sub-portion of the line to save. Overall, it was felt, and I agree, that mile for mile the Antonito to Chama portion was the best.
Why not just Chama - Osier? My guess is that, since this was a compromise in the first place - saving 64 miles instead of the hundred whatever - they took all that they could get. Also, the Antonito Osier portioni would be needed to connect the D&RG with Chama.
But, personally, I generally prefer the Big Horn - Osier stretch scenery wise, if not for any other reason that the RR is the only way to see it.
Don