I don't have the analytics but I would bet fair money the Haddad is the most commonly performed class 1 tuba solo. Haddad doesn't need to be class 2...it's 3 movements, 9-12 minutes long, the bookend movements are fast, it goes up to a high-C--it's an expansive work. It's the most recommended piece to give high schoolers because it's both moderately challenging and highly melodic (therefore fun to play). But here's the thing, if you compare it to similarly lauded works on the Flute, Clarinet or Horn lists, it's cheesy, campy trash. It's not a good piece of music. On the other hand, the Vaughan Williams is the most prolific work written for the tuba, and even it can be kind of campy in places. The whole first movement is a hulking, hobbling tuba player stereotype.
Aside from RVW and Hindemith, none of the "masters" wrote for tuba. It didn't even exist in their time for most of them (Mozart, Beethoven), and the 20th century guys just scoffed at it. So when 'Air & Bourree' got bumped down to the class 2 list, tuba teachers flipped. Not just because it meant their freshman would no longer get an easy ride to state, but because it meant the actual list of good pieces that can be played by ANY high schooler (not just freshman fresh off of the class 2 list) got reduced from, like, three pieces to two. That's a slight exaggeration but you get my point. Air & Bourree, despite now being a 2 is actually on many college entrance lists because there's some relatively good material in it. That should tell you how starved the list is, and how getting hoity toity with such a sensitive list royally SHAFTS these tuba kids.