With the brass instruments, it's more of a question of physics and the harmonic overtone series.

The basis overrtone series can be played on a bugle with no valves. It starts with a tonic note, then a fifth above that, then an octave, then a third, etc ..... much like teh drawbars on an organ. Drawbars duplicate the overtone series. (This must be printed somewhere.)

As it goes higher and higher, the notes become closer to chromatic than they are in the low registers. So, the overall tone and characteristic of a Bb tuba must have something to do about the way it handles the harmonic overtone series.

There is a limited number of C melody saxes floating around and they are HORRID. Some genious thought that it would be easier to read fake books, but the tonal change was too great. Just as with all things acoustical : SIZE MATTERS.
( only acoustical, now )

Tuba's BTW are in Bb.
It's all about tone color. There is a Bb and an A clarinet, just for those really hard keys. Serious orchestra players need them, but most casual players don't.

For many people, C is not the easiest to play in anyway. It certainly isn't the most comfortable fingering on a piano keybed, that's for sure. Brass players tend to prefer sharp keys, while wind players seem to handle the flat keys better. There are only 12 keys, so it can't be THAT hard to get cozy with more than just one, huh?
If you always play in the same key, you'll probably get carpal tunel (sp?)too.