If Smash 64 was developed right after the very first Mario Bros game and had Luigi as a separate character, I would have found it just as strange. But I don't think it's at all fair to compare Luigi and Dark Pit at this point though..
Key words "at this point".
Back when Smash 64 came out, Luigi was basically "Green Mario"/"P2 Mario" during a period of time when "Super Mario Bros." became "Super Mario" and leaving Luigi out of anything that wasn't a multiplayer side activity.
And yes, he is biased, because it's not like Dark Pit was the only option for an alt.. Of course at some point you have to make a decision and can't waste time making the clones, but I definitely feel that Sakurai went with a personal preference rather than choosing what he thought most fans would enjoy.
We get....
Genderswaps for Wii Fit Trainer, Villager, and Robin
A wireframe design
and a hoodie for Little Mac
Two sets of shorts and sports bras for Zero Suit Samus
Alph as an alt of Olimar
All 7 Koopalings for Bowser Jr.
The obvious overalls for Wario ala Brawl
Whatever alts we don't know about yet (such as Dry Bowser, Metal Sonic, Ms. Pac-Man, etc.)
and Dr. Mario and Lucina as another two intended alts that were promoted to last-minute clones due to gameplay properties
Yet Sakurai is biased just because he even
considered Dark Pit as an alt to begin with.
Despite....Dark Pit being considered
an obvious choice as an alt by quite a lot of people and being pretty much a
requirement with Pit's outdated black palette.
So we know that the "Dark Pit wasn't the only option" reasoning to be moot considering he was one of
many alts and held more merit than say...Alph.
I won't lie though, I guess I'm just biased against characters with such lazy designs. I just wish game companies would just put to rest the whole 'dark' character concept.. Leave the creating a new-character-from-recoloring-an-existing-one to the fanbases already. I'm always reminded of
this video whenever I see such characters.
I agree that dark counterparts are a cliché at this point, but at the very least Pittoo defies near all the tropes associated with them and is a breath of fresh air. And isn't something like "Mecha-Pit"....