C'mon now, lets all admit it, sex and sexy material sells, its simple as that. Like ace says, the target consumers of the game is young and adult males, what better to get their attention with than sexy ladies? Muwehehehehe
But lets also not forget, Soul Calibur is a very well balanced Fighter game and it'd sell regardless of the "enhancements" of the characters. Its high-quality graphics and the simple yet in-depth controls are just some of the things that make it attracting, its just not the sexy ladies in skimpy outfits, thats more in the lines of the Dead or Alive series...which by the way isnt half bad as a game either. Hehehe. To add they've outdone themselves with some of the guest characters and designers. One such would be Yoshizaki Mine's (creator of series like Keroro Gunsou/Sergeant Frog) Angol Fia.
-You can find a couple of screenshots and Yoshizaki-sensei's art of her on the HP:
Yoshizaki Mine's HP