The main reason big boobed actresses are shunned in Hollywood is this: the rise and dominance of female casting directors. This has become an overwhelmingly female occupation.
By the time Puritanical standards were beginning to be relaxed in movies (late 1950s), women began taking over this profession.
There is hope, though. TNN is ramping up their "Network for Men". I couldn't imagine them turning down a beautiful, talented woman with giant hooters as a host of a nightly talk/interview show, for example. I don't think they'd pass on a detective show starring such a woman, either (e.g. VIP with bigger boobs).