Why do people on this site think Splatoon is going to do so well?

[Hynad]

It's going to do well in relation to Wii U's user base. But it won't move that many consoles.

Not all games need to sell 3M copies to be seen as doing well. It's all relative to the developer's financial investment. Not all games need to sell Grand Theft Auto levels to recoup and turn a profit.