Probably because there are already Adult. and Adults as bands.\
Just a way to have a name that is easy to google I suppose (same with other alternative spellings). IE Waavs instead of Waves. DIIV instead of Dive. BLVCK CEILING instead of Black Ceiling.
Its just a throwback to Latin, where the letters are interchangeable. Other languages Call W (translated) Double V instead of Double U.
Or they are just trying to be difficult.