Let me ask this question. How many of you have been to a "local" marching band competition outside of Texas, California or Indiana? I state this as someone that has gone to many competitions throughout the Mid-Atlantic over the years. A day long competition tends to be broken up into multiple segments. The smaller classes in the morning/early afternoon and the larger classes in the evening. Even regional shows tend to fall into this same break-out of classes.
Most of the "Marching Band" shows I have been to over the past couple decades have two groups of fans. The first group of the fans tends to be the parents of whatever band is performing at the time. The second group are all the band students that have already performed and are waiting for scores. If you doubt me, go to a competition right before the first group of scores are announced and watch the mass exodus once the score are announced. They don't return and the sequence starts all over for the next group of classes.
"Drum Corps" shows tend to draw a different fan base. This fan base includes alumni as well as fans of Drum Corps. Many spend some time in the parking lot and then go to the stadium for the entire show. They tend to drive longer distances, stay for the entire show and spend more money. Often these fans have no ties whatsoever to the competing corps they are going to see at the local competition. That is what makes Drum Corps unique. Once it becomes a marching band, it loses that uniqueness and subsequently, its fan base. I have gone to many drum corps shows over the years that did not include a single corps I had any ties with whatsoever. I can't say that I have ever been to a marching band show where none of my kids were marching or I had not taught at a competing school.
I have driven or flown to dozens of drum corps competitions over the years. I doubt I would do the same for a marching band competition when I can see them locally with schools I have taught at in the past.
As I stated earlier, you can see a marching band at any local competition. The desire to go and see one that is the same thing, only cleaner is not as big a draw as going to something as unique as Drum Corps.