I think it's that 'Boba Fett' appeal of the character in that we don't know much about him, so we therefore want to know more. A bigger, starring role could flesh out deeper origins for the concept of this dark doppelganger and perhaps could connect to other lore from older titles.
It could also perhaps be because other Nintendo 'dark' characters have held bigger spotlight roles in their own games. Dark Link may have been the final battle in AoL (I'd say spoilers, but it's been said in this thread above, and c'mon, it's a 25 year old game that no one seems to want to bother trying to play.) but he / it hasn't had a big starring role like Dark Samus in the Metroid Prime series, or the Shadow Mario that stirred up trouble in Mario Sunshine. Dark Link, or Shadow Link, has been reduced to a cameo role / mini-boss battle ever since.
I also think, because of people's general feelings on the overuse of Ganon, and the displeasure of the direction of new villains that they will find fleshing out someone they already know exists will be more fulfilling. Taking something that's somewhat unknown and making it known is a popular notion in and media even though in the end, most people end up disliking how known they become. (See Boba Fett, again.)