As you and Stumpel have pointed out there is a pro-allies bias with a lot of players (myself included) both in low-player matches and high-player matches. This bias inevitably unbalances the teams; at first for low-player matches, and over several matches for high-player matches.
I don't think it is a distaste for the Axis side, but rather a preference on the part of many players to be on the attacking side, especially for those who value objective play.
The solution, therefore, seems simple: ensure that there is a balance of maps - as close to 50% of maps as possible should allow Axis to attack; meaning both teams get to attack and defend with greater regularity.