Partially because game based arguments are entirely subjective ...
In my opinion Halo 3 and Mario Kart Wii both have a similar problem in that they are not that great as single player games but they shine as multiplayer games. Being that I like to play multiplayer games with people in real life, and I only enjoy most FPS as single player games, I would say that Mario Kart is dramatically better than Halo 3. At the same time most reviewers have an (unhealthy) focus on online multiplayer and advanced graphics, and these are a few of the reasons why Halo 3 received a much better review score. We can argue back and forth over which is the better game but it will likely just result in us having to agree to disagree.
In contrast, sales are entirely objective and it is very easy for me to say that Mario Kart Wii has sold better than Halo 3 and is one of the fastest selling videogames of all time.
Quem disse que a boca é tua?
Qual é, Dadinho...?
Dadinho é o caralho! Meu nome agora é Zé Pequeno!