You need to take the nostalgia glasses off.
Regardless of what you felt at the time, the truth is, you cannot compare the technology and how the world works now vs 2004, the amount of comments, shares and likes on facebook alone, would trump the buzz around Halo2 on release date everywhere on the web.
Halo 2 sold aprox 8M copies, it was quite big for the time, but sales have been dropping ever since, with Halo 5 selling around 5M copies, and MCC 3.13M, so to imagine that this one will sell about twice what its most sold game sold, is ridiculous.