oops, forgot about those links there and are outdated.Interesting site in your signature Steve, tho a couple of links are brokenThe Bb clarinet is the easiest to handle for it's size and is the most preferred for it's tonal characteristics.
A C or Eb is pitch-wise hard to control except for experienced players and the tonal characteristic doesn't blend well with other "starter" instruments. And the tone is harder to control for a beginner.
You don't see many brand new clarinet players on an alto or bass clarinet, or even oboe or bassoon (yes, not a clarinet but they usually snag a clarinet player for these). Oboe and Bassoon requires a more advanced player in a sense. Plus how many 5th graders in the US can handle such a long instrument like a bassoon. But then over time the Bb clarinet has been the dominant instrument in bands and orchestral composers largely write for the Bb & A clarinet due to it's tonal quality. An Eb clarinet is high in it's tonal pitch but the Bb's tonal quality is preferred with the As.
If you look at the clarinet family it has one of the largest spread of instruments (of course, one of our experts here will show off his saxophone line). But you can find clarinets in G, F, E, Eb, D, C, Bb, A, Ab and probably everything else if you search around long enough. Handel's 1748 Overture required D clarinets due to it's tonal characteristic that he wanted with the rest of the orchestra. And Mozart wrote some popular pieces for the Basset clarinet in A. So the writer is looking for certain tonal characteristics, they reason instruments have various flavors .. such as the tonal difference between a Cornet and a Trumpet.
One reason the alto sax is the starting instrument rather than a tenor. Just think if you starting band had 8 soprano saxophones and what that would sound like ? Or how much larger and heavier a tenor is from an alto for a young player ?
My main saxes are Selmer Paris SA80 soprano, VII alto & tenor and Couf Superba 1 alto and tenor.
Clarinets: my primary is a Buffet RC Prestige, Buffet R13 Bb/A, Leblanc LL Bb/A and various others.