It's said because of many Nintendo fans' expectations. I was ripped last year by multiple Ninty fans when I said it wouldn't sell as much as the DS. I noticed it on other memebers too. Henceforth, some people say it's doing bad because it's tracking below the predecessor.
Personally, I think it's doing well. Just because it's selling less doesn't mean it failed.
e=mc^2
Gaming on: PS4 Pro, Switch, SNES Mini, Wii U, PC (i5-7400, GTX 1060)