They sometimes attribute this to incompetence or venality on the part of the judges. It is not true for a one thing. You may run across a less than ideally astute judge, but only very rarely. Contest organizers seek out the best judges available. Venality in judges is even rarer.
Judges are honorable.
If they weren't honorable they could certainly find a better way to exploit being dishonorable than judging marching contests. Politics comes to mind. There is a good reason why the great criminal masterminds have never paid any attention to band. There is not much money in it. By the time bands get to contest, band parents' bank accounts are too depleted to fund attractively large bribes.