It is actually really simple: Unlocks and usability.
In GW1 I could buy a costume set, which was then unlocked for the entire account, allowing infinite free respawns. I could have my entire account in Halloween clothes for Halloween without having to pay extra or sacrifice precious storage space to store the outfits ’till next year. This elicited the “gotta catch them all” response for me, and thus I bought all the costumes, even the ones I never even used, just to have them available. In GW2 on the other had, I have to weigh each purchase on a (literal) gold scale; only buying items I really want and that I deem worthy of the storage space they consume (which is at a premium already). No more buying items just in case, or for a bit of temporary holiday fun. And to the surprise of no one, I have yet to buy a single costume item.
Usability is of course the fact that so many of the items are only allowed as town clothes. Now I understand that it became a bit silly in GW1 with people running around in DoA in top hats and evening gowns (even though the line is blurry to some of the actual armor sets… :P), but I would still like to use my sunglasses everywhere, not only in town. If at least there was a way to set it so that town clothes were automatically loaded when entering a town it would be immensely better. But as is one has to make a conscious choice to activate town clothes every single time entering a town. So people just don’t bother (surprise!). Then of course the question becomes, why buy it in the first place?
The same basic principle of not offering items in a format most convenient and enticing for the customer (another example is random dyes only, not even sorted on hue apart from green and orange) goes for a lot of the GW2 Gem Store items, but where ANet really lost the money on me was the costumes.
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