I agree that there's been a slight bias against the Cowboys regarding the HOF. Not necessarily widespread, but because of the limited number of voters it doesn't take many voters with a bias to screw things up.
But I also understand why there has been a widespread bias against Cowboy fans. Just spent an hour listening to some idiot Cowboy fan blowhards bitch about how they didn't get 3 in this year while at the same time trashing Warren Moon as completely unworthy because he didn't win a Superbowl and just got in because he was black and padded his stats in Canada. Nevermind that he led inferior teams to numerous playoffs, 51,000 NFL yards, more than 300 NFL touchdowns, top 5 in 5 NFL categories, and was selected to 8 straight pro bowls.
"...trashing Warren Moon as completely unworthy because he didn't win a Superbowl..."
That's just ignorance. Moon was a good QB who played for a mostly mediocre team. There were a few other good players, but not enough to win the big one. Coaching and drafts also played a role. There are many factors that must all come together at the same time to make it to the SB and win. It shouldn't keep a player from being recognized for his talent and accomplishments, but often, it does. Perhaps that's why football is called a team sport. Now, if someone thinks Moon should not be in the HOF for other reasons - like other more deserving players, or lack of stats, or lack of leadership, etc. - then that argument should be made.