Interesting though...
At another level the question is rather clever - why indeed is there a semitone between the 3rd and 4th, and 7th and tonic notes ?
Basic harmony from the overtone series can give us the tonic, 3rd, 5th and b7th - and therefore we can derive intervals with the tonic and its overtones - I think somewhere that is where the major 2nd interval comes from.
Now we have those intervals its just a case of fitting major 2nds into the gaps in a "pleasing" way - I think and thats is where there may be argument - i.e deciding what is pleasing to some cultures may not be pleasing to others and over time some sequences become standardised, like our own major\minor scale series:
Major scale Tonic, T,T,S,T,T,T,S back to tonic again
The start of the minor scale looks like Tonic, T,S,T... - just the alternative way of fitting the maj 2nd interval into the gap between the supertonic and the perfect 4th.
As for naming them C, D, E etc - I've no idea, but I suppose they have to be called something....
I've probably missed some stuff in all this, but basically thats my understanding! (oh, I haven't mentioned Just and Equal temperament as it may start a nightmare post....)
Actually - now I've had time to look, there is a good overview page on Wikipaedia - and that major second is there as the 9th partial.
http://en.wikipedia.org/wiki/Harmonic_series_(music)