I think the #1 reason is that CATS and the D&S are left. Period. I don't think the D&RGW would be anywhere near as popular if it wasn't for these. They are two of the longest heritage railroad operations in the world...along with excellent scenery and large rosters of equipment.
It is the late 1930s-1960s D&RGW that most people love; it was a mere shadow of the D&RG from before. Look had how many fascinating pieces of rolling stock it used to have! Look at how much larger it was then! Instead of just 2-8-2s and a handful of 2-8-0s like the 1940s D&RGW, it had 2-4-0s, 4-4-0s, 2-6-0s, 2-8-0s, 0-4-4-0ts, 0-6-0ts, and 2-8-2s. Clearly, there is little interest in the D&RG, as most interest is in the D&RG
W. After all, how many threads do we have regarding the Mountaineer, the myriad of moguls, or the Pullman service? How about all the interesting connecting lines and interchange?
My interest in narrow gauge started with either my dad's 8mm films of the CATS or his handful of On3 models. Most models and films are based on what is left...mostly D&RGW. It doesn't seem that models of Class 60 2-8-0s sell very well, yet tens of thousands of C-16s are sitting on model railroads throughout the world...nearly all of the models being based on the last 4 D&RGW C-16s...especially the two that survive in Colorado. Most of the testimonies on here and elsewhere don't start with how fascinated they were with the XYZ narrow gauge until I discovered the D&RGW...they start off with "I went to Chama" or "my dad took me to Durango" or "I saw a K-27 at my local hobby shop".
Is the D&RGW an interesting railroad? Yes...it wouldn't attract so many fans into the narrow gauge tent if it wasn't a very interesting railroad...but I don't think the role of the CATS and D&S can be overstated.
Michael