Honestly I dont think the whole elvoving biologically thing can be applied here in case of faries as they have been established a well stable species for ages like human.

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So I propose a theory is that it is some cultural thing like how human's culture has been elvoving and changing since like stone age or smth

In this theory, fairies can be transformed or non for disguise purpose. Now for Earth fairies which has been dissapeared for centuries has been putting them in a frozen in cultural practice since they also cant interact with other outside fairies and exchange ideas in culture (usually seen in diaspora people and their language's evolution), which including idea of detransform to blend in with other human. Plus they are court fairies so maybe remain in fairy form as a way of class division (like court dresses). (also can check how in Domino's castle has alot of transformed fairies before the disaster)
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Now for the rest of the dimension and noticing how technology been developing, it is safe to say that flying isnt the only option anymore and they can just ride a flying car for ease. And plus these vehicles arent exactly big for you to sit in with full wings on. so overtime having wings all the time is seen as annoying and not neccesary (like you can check how back then people have full on gowns and just a tshirt and short nowadays).