KuroKitsu
Headphoneus Supremus
The way I see if is that a lot of people compare iems to 2ch. That is their "reference" is what they hear produced by DD speakers. Given that BAs are different method of reproducing sound, there are bound to be differences in that representation and therefore sound "off" in comparison. Like you said though, objectively speaking BAs can produced a wide variety of sounds in a natural way.I'm also not implying that it's got a plasticky timbre, which - for some reason - a lot of people tend to stereotype with all-BA IEMs. Given the sheer variety of sounds I've heard come out of BA's, and how natural and meaty the A18s sounds, for example, I have no clue how that association ever came to be, but I digress.
It's probably like the whole copper = warm, silver = bright mentality with cables.