Because if CO did that, there would not be more people "choosing" the middle class section, but there would be more people DEMANDING that they be seated there for no extra charge. ("I have a medical condition and need to sit there" - - "I am going to start flying CO all the time - do you want my business - if you do, prove it, and seat me there" - - "I have Ultra-Plutonium status - my 4 kids should be entitled to those seats for free" - - I had status last year and only missed it by 12,000 miles this year because of my wife's sister's uncle's brother and his cancer surgery.....but now you are punishing me because of his health problems...and you should give me that seat". My guess would be that if CO had a mid-class section, they would give away 75% of the seats with no revenue collected for them.