This goes back to the early days of "modern" instruments. Initially, instrument-makers did not have the accuracy of instruments (mechanical) to create keys, accurate boring, etc. If you look through the history of any wind instrument, you'll see such profound comments as "and then it received two keys!" Because most instruments were created at a time before using keys, you had an instrument in a single scale degree. If you had a C clarinet, you could not play F#s or Ebs. You played diatonically, in the C scale.
As instrument-makers learned more precise tools and were able to create more complex systems, the number of clarinets needed to play the full range of scales diminished. We were left with three soprano clarinets: A, Bb and C. Bb sounds the best, and was determined to be the winner of the clarinet races. It is most commonly played. However, any professional clarinetist has all three "orchestra clarinets", because there are parts still written for them.
Part of the problems with the other instruments: the modern A clarinet has been designed to fit the Bb mouthpiece. Unsurprisingly, this is a less than perfect match. It means that the A clarinet has interesting intonation problems and what's commonly referred to as "the grunt" -- a sort-of grumbling sound from the instrument in the second half of the clarion register. It just doesn't want to play them!
The modern C clarinet on the other hand tends to be a bit brighter (is smaller) and also has different intonation problems than the standard Bb. I think the C instruments are not bad, but because they aren't standard, harder to find essentials like reeds, ligatures, replacement parts, etc. Not hard, just harder.
These instruments are only commonly used today in orchestra. In bands of any variety, the Bb is commonly used. It's not a difficult transposition, and trying to "re-write" for concert pitch would probably induce screams as loud as asking a violist to play in treble clef. :-P
The long and short of it is: there are more than one clarinet, there are reasons for playing any, and those reasons probably have more to do with tradition than anything else.