My guess would be twofold: Aesthetics and Ergonomics.

Aesthetics is self-explanatory... less bulge in the area between the cups makes it more sleek (and ***y).

Ergonomics because, you cannot put adjustable latches/hooks up front as this would change the distance between the cups. The said distance should remain constant to maintain the proper breast profile (just made that up) even if the wearer needs to readjust the bra's tightness for proper fit.