Many marching bands actually play tubas in the US too, even high schools where some freshmen can make the tuba appear to have legs. 😆
The Sousaphone was/is satisfactory for the simple marches, but in the very wide range of music often played on fields today, rock and blues, jazz and symphony excerpts, there is no substitute for the full throat and octave range of the 4 key B-flat Tubas. Then again, I’m speaking of areas that are capable of fielding a large band, often with the complimentary orchestra percussion section remaining on the home team sidelines, while marching drums remain embedded within the complex field formations.