Because he is stating it like it's a fact and not an opinion / prediction.
It cannot be anything but a opinion/prediction, no matter how its stated or how right he thinks he is.
Spock is jumping the gun but Chris's assumption that Wii U's situation hasn't changed is disingenuous at the very least. His assumption that Wii U won't sell any different from last year is based on Wii U's market situation being the same, however between the "killer app" marketing, desire of hardware outside of launch and the lower price and bundle. Wii U's situation has changed, he might be right but his reasoning is flawed so speaking in absolutes is a bit arrogant imo.
I agree that the Wii U will do better than what Chris predicted.
Had he predicted higher Wii U numbers, do you think he would have received the same sarcastic crystal ball comment? Probably not, as there already is a poster in Spock who is also making all sorts of assumptions and is not getting the same reply in return.
Surely it is "okay" to predict differently than some and not be treated as a lunatic with a crystal ball/DeLorean car/Aladdins magical lamp