Okay, it's been an hour, we can stop ignoring Metal Overlord now. Carry on.
Y'all haven't totally ignored him yet=???
(jk, Metal Overlord is a cool son bro man dude!)
Do we not consider Chibi-Robo because no one is asking for him. Chalk it up to an oversight, or unpopularity? Or just, no one really wants to see him in Smash? Because how could you NOT love that little guy?!
Chibi-Robo fits into the "modern 'WTF?' character" category, and so far, aside from Pichu Roy (and arguably Ness in Smash 64), we haven't really gotten one of those. At least Pichu was popular, Ness' game was somewhat popular in Japan (and has reached cult status worldwide) and was hugely influential on gaming (its' biggest influence: Pokémon).
I think Chibi-Robo could definitely make an interesting character to randomly appear. I just have troubles seeing Chibi-Robo getting in over newer characters such as Starfy, Bowser Jr, Victini/Zoroark/basically every "popular smash bros choice" Pokémon, Krystal, Issac, Saki (well, he's new to all of us outside of Japan who never played the Japanese version of Sin and Punishment), etc. That's a stiff field to go against. I have a hard time seeing Sakurai or really many people loving Chibi-Robo over most of those choices.
Personally, I would definitely prefer Starfy, Bowser Jr., Victini, Issac, and Saki over Chibi-Robo. I've got nothing against Chibi-Robo, but I think the lack of having any games well-received outside of small markets or groups of people not just internationally but in a country-to-country basis, which includes Japan. Now if Chibi-Robo's series had a more appeal, better sales, a more "cult-like" status (viva Las F-Zero, Mother, Fire Emblem, and Kid Icarus) or a bigger fanbase, I think Chibi-Robo would definitely have more demand, and would be a more desired character as a potential smash bros newcomer.