My wife is currently in this situation. She's not a new job seeker by any means; she does mid-to-high-level management, leading complex projects. She recently applied for a new job that she's really enthusiastic about, but with them not responding, she's rapidly losing interest. She would be a perfect fit, but lack of a timely HR response could cost them a good employee.

Of course lack of a timely response also reflects badly on the company, so that could also be a very valid reason for people to lose interest.