A few reasons stick out to me.
#1 - Brady did so well against Washington in practice they are happy where he and the WRs are currently.
#2 - They are shopping Mallet
#3 - The field is notoriously bad
and all of those reasons I think matter. However I want to look at probably the most obviously reason that has not been talked about.
#4 - Does Mallet warrant a roster spot?
The reason you carry Mallet this year is not just if is better than JG. The reason you carry him is he you believe he is a true quality back up QB. You need to honestly find out if you can go .500 with him over a 4+ game stretch.
If you can then you keep as he adds the value of keeping you in the bye hunt if Brady goes down for a large part of the season. However, if he can't do that no matter how much better he might be than JG what value does he have? If one plays better than the other but neither can keep you in the playoff hunt cause they are not good enough then why keep both?
May as well cut mallet and try to find someone else or just admit your hopes rest on Brady to not miss much time. I think this and game 4 will be the games that determine if we carry Mallet or not (assuming we can not trade him).