The problem is lack of competition.  Should a reputable exchange with good customer service (or at least somewhat good customer service) pop up in the US, I'm sure many users would jump ship in a heartbeat... and not just from mt gox but from just about every other exchange.  This may be the greatest opportunity for a reputable wall street investment bank or other large, reputable company with the resources to tap into a wide open market where users would flock in droves. 

Most likely the only thing holding them back is the uncertain regulatory environment, with precedence being when the government shut down mt gox's subsidiary mutum sigiliam on a technicality and confiscated all assets. 

Opening a reputable US exchange would be the business opportunity of a lifetime.