Before this week's dismal performance, I think Mason had more receptions than any other receiver in the league.
McNair is small. Also, he is the #1 receiver. Other teams are doubling him/rolling coverages toward his side of the field and forcing McNair and other receivers to beat them. With George on his last legs, Mason is the #1 option on the whole offense (after McNair), so other teams are not surprisingly taking him away. Perhaps if McNair keeps exploiting other areas, opportunities will open up for Mason.