Of course the answer is, all of the above; and most likely myriad other reasons broad and personal that we haven't delineated yet. As deeply curious a species as we are, we also have a seemingly hard wired system within us that makes sure we don't get overwhelmed by something strange or threatening. In this case it manifests as a blanket trivialization of attire; and of fashion in particular.
Where I'm going here though, is an attempt to partially answer those who routinely dismiss the importance of what goes in the world of attire. If we can accept as an essential truth that one of the primary goals of attire is to create a symbolic set of communicative iconography, a language if you will of garments and accessory items that help us establish to ourselves and those viewing us some aspect of our nature, then this problem becomes slightly easier to address.