Everyone went to those low, boxy type of stays like the Merida or S-Works because of aerodynamics. The shape is good for air flow around the seat tube, and they are the lowest height that is UCI legal (otherwise CF aero bikes would have no seat stays).

The other shapes are largely designed to look comfortable or cool. But it is true that if you want the chainstays to flex but not be paper thin, small diameters work best.