Because back then corps didn't have exclusive endorsements from horn manufactuters like they do now. Why are they called tubas and trumpets? The answer is because king,yamaha,dynasty and everyone else wants them to be called that. This makes it easier to market these products to there target audience; High School Band kids that want to play on the same instruments that their favorite corps uses. Kids who have never marched corps don't know what sopranos and contras are, however they do know what trumpets and tubas are. It's marketing plain ans simple.