our collective nightmares from the 2010 World Cup Wikipedia, the vuvuzela is a monotone instrument; it famously plays (roughly) the B♭ below middle C.
Why is the vuvuzela restricted to only this pitch, while non-valved bugles can play multiple pitches within the harmonic series?
My question is very similar to Physics behind why a bugle can play several notes, while a whistle only plays one note, but the answers there focus on the fact that a bugle player uses variations in their embouchure and air speed to create different pitches, and these variations are not possible on a whistle.
Yet a vuvuzela seems to have a mouthpiece much like a bugle, which would suggest the ability to have these embouchure and air speed variations and therefore the ability to create different pitches:
So there must be some other difference with the construction of the vuvuzela that prevents it from playing other pitches. What is this difference?
As a subsidiary question, brass players like to say that the instrument only amplifies what they do on their mouthpiece. So if we rigged a trumpet mouthpiece to attach to a vuvuzela, does that mean it could become a multi-pitch instrument?
(I know that this question might blur the line between physics and music. But since such gray-area questions are often allowed here, and since we've already allowed a similar question, I'm assuming this is safely on topic.)