Clearly the actual demand base for a title like Mass Effect simply isn't huge on 360. I mean even a 10 million selling title is only actually appealing to 20% of a 50 million console install base.
Reviews, etc only matter where the demand is there. I'd always figure a title like Mass Effect simply has a much lower ceiling than something like CoD or Halo on 360.
It's not like every game has some huge potential that goes untapped. A lot of titles have ceilings based on the demographics on the console.
Try to be reasonable... its easier than you think...