||I think it might be due to the process being somewhat unclear. Also, I don't have the statistics on this, but it's been my experience that the majority of active topcoder's are university students who might not be actively looking for positions.
As far as the process being unclear, there are two major points I can see. One, is getting knowledge about it out there. TopCoder is becoming an increasingly complicated site as you offer more services. For example, one of my friends, a frequent competitor didn't even realize there was an educational content section, a design/development competition or this service that you are providing. Getting the message out is key, I'd suggest at least pointing people towards it during SRM's. Of course there is a line at which people get annoyed with the service being advertised, but for example, adding a quite noticable image somewhere to the header of the site if you haven't signed up for the service might be an idea. Of course have an option to disable it if the person doesn't want to sign up for whatever reason.
The second big reason is lack of transparency in the process. Sure there are 15 companies there, but before this, I had no idea what kind of positions or salaries they were. Are they looking for fresh college grads, or experienced veterans? If they are looking for both, let people know that anyone and everyone is welcome to apply. What steps will I go through as a person who signs up for this service (which I have btw, I had signed up for it and I still forgot about it until TopCoder brought it back up recently)? How quickly can I expect to get a call? How many jobs are open? Etc, etc. I know some of those questions are hard/impossible to answer, but any information helps.