Because "it's hard, so quit" is how to best get places in this business.

Problematic scheduling can be overcome by thinking outside the box.

OP: Consider having a beginner band for a year or two at the high school level. Get some kids in that never had a chance in middle school, or that may have dropped before 9th grade. They'll learn faster than you think, may be willing to march (even if they don't play) and will be a future asset.