Because people, or at least a lot of them, hate winners. And people are unwilling to concede the fact that the Patriots have been an excellent and well coached football team for a decade or so now. I mean look at MLB. Who outside of the Big Apple likes the New York Yankees? Because they have been consistent winners for like forever (and I'm certainly not a Yankees fan, but that's the truth. They are struggling by their standards this season, and they are still going to make the playoffs).
Check out my user name. I'm a lifelong Bruins fan in the NHL, so clearly I hate the Montreal Canadiens (forever). Because their historical success cannot be disputed. Hell, look at John Cena. One of the best of all time, and look at the negativity that consistently surrounds him.
People are quick to accuse the Patriots because they resent their success, so therefore they are looking for reasons, anything, to throw cold water on it. And that's why there will always be a black cloud over their heads, and people clamoring for an asterisk next to their name.