#1
You see, tuba is first and formost a fun fun fun instrument. And the people that play it are the coolest people ever (no, were actually not fatty fatties)

And its actually a melodic instrument when someone whos good plays it (aka, not a highschool student)

And last but not least, since no one plays tuba, its really easy to make a living playing it.
#2
It's a bitch to carry around all the time on the off chance that a fat gentleman might walk past.