The reason is THE BRAIN. THe brain likes frequencies that are simple proportions. It thinks they go together. You should really ask, first, why are there octaves? 

Well the octave represents a doubling / halving of hertz (cycles per second).

So, midi middle C is 256 hz, and if you know your computer numbers, you'll realise that the next octave C's are at 512, 1024, 2048, etc and the lower octaves are at 128, 64, and (pimp your ride) 32. 

Earthquakes, by the way, show up at around 11 hertz.

Every society starts with the octave. 'Cos 1/2. Got it? 

(I propose that the 2nd Viennese school abandon the octave by the way, and also tuning the instruments. Niether make any sense for them. The current state of things with octaves and tuning and suchlike is pure hypocrisy. Let it go, boys! Also scores. And playing in public. Nobody comes anyway.)

Hh HHm...

How to divide the octave?

If we start it on C and divide it into 3 (which is a nice brain-friendly proportion) we will get a lovely 3 note scale:

C, E, G#, C 

How about dividing it in to four:

C, Eb, F#, A, C

"THat's nice", says the brain, "but it's too SYMMETRICAL. Both of these scales just seem to go on forever and ever, I can't tell what's what. I know! Why don't you mix and match the proportions so they are slightly more uneven? Then I can figure out the bass note.".

And thus was born the "Proto Major Thingy":

C, E, G, C 

and the "Proto Minor Thingy":

C, Eb, G, C

"Hang on a bit", says the brain, "you missed a note, didn't you?".

"Where?"

"Between G and C, I'm pretty sure you had something between G and C".

C, E, G, A, C ?

"Thas NICE! Rock and Rollish. Go on then, what about the other one?" 

C, Eb, G, Bb, C ?

"Hey, what's with the Bb? We never heard that before. What kind of proportion is that?"

"It's 10/12ths".

"You mean 5/6ths. Alright. Play it again".

C, Eb, G, Bb, C 

"Kay, that's bluesy. All right! But its 70,000 years ago and there's loads of poor bastards buggerin' round the the scenery getting crunched and munched by sabre tooth tigers and suchlike. Lotta funerals. Mucho sadness. Like Trump nowadays, you should know! Need variety. "

"Permutations?"

"Show me."

C, D, E, G, A, C<br>
C, D, E, G, Bb, C<br>
C, Eb, F, G, Bb, C<br> 
C, Eb, F, G, A, C<br> 

"Whats the F proportion?"

"4/3"

"Great! More, now there's chieftains, mud huts, jewelry"

"I need some rules".

"kay. Er.. keep the Minor third or the Major third and the Fifth where it is, and just move the others about... I know, like this: move the seventh up,
the sixth down, the fourth up, and the second down!"

C, D, E, G, A, C<br>
C, D, E, G, Ab, C<br>
C, D, E, G, Bb, C<br>
C, D, E, G, B, C<br>
C, Eb, F, G, Bb, C<br> 
C, Eb, F#, G, Bb, C<br> 
C, Eb, F, G, A, C<br> 
C, Eb, F#, G, A, C<br> 
C, Db, E, G, A, C<br>
C, Db, E, G, Ab, C<br>
C, Db, E, G, Bb, C<br>
C, Db, E, G, B, C<br>

"Hey, if we superimpose them all we''ll get 12 subdivisions! Brilliant!"

C, Db, D, Eb, E, F, F#, G, Ab, A, Bb, B, C

"That's why I'm called the BRAIN, son. Oh, and you're welcome."