I would've put the Triports I heard at about the $50-60 range. They were better than most of the cheapie phones you run across but not worth the asking price.
Anyway, my guess is that it's a combination of factors. Probably that they don't care, don't understand why they should care, and don't have the means with which to quantify differences. Most people who have listened to my headphones will say they sound "good" and some will admit to being somewhat amazed or that they sound better than xyz. But they can't say in what way they are good, and they're not willing to spend similar amounts for similar quality. What they're getting isn't worth the outlay to them. They don't have the time and energy to really closely examine and understand what is available, which is why brand names and advertising work fabulously. People like Starbuck's not because it is the most fabulous coffee on the planet, but because it has a reputation and is expensive. They might be able to tell you that a cup of Starbuck's tastes better than a cup of Folgers, but I'd be willing to wager they can't say specifically WHY they like it better. Can't give a definitive analysis of the flavors or drinking experience that explains why one is better than the other. Same with audio. They haven't got the tools or just don't care.
Here's an example. It seems a lot of people on this forum like nice watches. I like Timex and Casio. Wanna know why? 1) They cost 20 bucks. 2) They keep good time. 3) They're nearly indestructible and could last decades with battery changes. Butbutbut! you say. There are finer timepieces out there! They've got fancy features, are works of art, fantastic jewelry, and appreciate in value! But I don't CARE about those things. For me, a watch tells time, and a $2000 Rolex does not do that job "better" than a $20 Timex, regardless of the fact that it may be a better watch. For the vast majority of people, headphones (or audio gear in general) play music. Sony headphones are cheap, look cool (arguably), and play your songs. What more could you want, asks Joe Public?
Most people listen to music for the content, rather than the quality or presentation. I'm not an artist, so if I look at a painting, what I can tell you about it is what it's a painting of, what I think of the colors used, and an overall impression of the piece. I have no clue (and sometimes no cares) about the overall style, painting technique, works it may be derivative of, medium used, or anything else of that nature that a trained someone might be interested in. When your average guy or girl puts on the phones and fires up the iPod, they're not thinking about the presentation or the sound quality - to them it's not about that, it's about the SONG. They either don't get that better gear would make their songs sound better, or simply have an understanding that it wouldn't increase their enjoyment of the music (or at least not to a degree commensurate with the cost). Which is understandable.