I think a lot of fans look at 125 lb fighters the same way they look at the WNBA. When they watch a professional sport, people want to be in awe of the size and athleticism of the athletes. They don't want to see something they and their buddy from down the street can do, like the power lay-up fest that is the WNBA.
It's no secret why heavyweights and light heavyweight fighters get the most attention. People want to be impressed physically by the fighters, not watch somebody the size of your 12 year old kid fight. People want to see a rare athlete built like a Lesnar or an Overeem.
I had a feeling this would be the case when UFC started adding all of these lower weight classes. You can't expect fans outside of the hardcore MMA crowd to get into those classes. And guess what UFC, the casual MMA crowd is what you are trying to rope in.