I don't know why they would do it this way. Wouldn't it be better to lower the importance of outliers? Sometimes you get a game getting all 7s and 7.5s and suddenly you have a 2 thrown in there or a perfect 10. At least then you don't have people at meta critic deciding which sites deserve more weight.
Why shouldn't outliers matter or be included? This isn't a scientific, objective study. These are opinions. Just because someone's opinion dissents from the majority doesn't make it any less valid.