While I'd rather see 5% than 2.3%, the numbers are not THAT discouraging.
I think it is important to differentiate between undergrads and those a bit older. If I was just graduating and had a big debt-load, I'd probably be looking for something in the private sector with big-time compensation too. Once you have a mortgage and kids, though, one of the things that appeals to you most is not necessarily travel, or promotions that may transfer you across the country, but rather the stability of the job, the extended benefits (for family), and the security of working for an employer who is not leaving town or being outsourced to Mexico or India. In other words, those aspects of federal employment that are usually strong attractants are not necessarily strong attractants to the age group surveyed. You will note that 17.7% indicated working in the public interest in some capacity. That may be what they want and can afford now, and it may well translate into a desire for something more substantial and secure. The romance of NGOs can wear off, sometimes.
Second, while there is certainly federal representation just about everywhere, and the schools surveyed seem to be situated everywhere, do not ignore the proportion of students situated where the jobs aren't. If the survey was confined to universities within a 50 mile radius of DC, I would imagine the proportion pondering federal work would be much higher. For a chunk of those surveyed, federal work may simply not be on the local radar.
Finally, there is something to be said for the pervasive perception that the public sector is either in the process of being scaled back, or under threat of being scaled back. I know that in the Canadian context, federal job postings have reduced in number over the last few years, and applications have also dropped substantially. The lastest federal employee survey results indicate that promotion rates have reduced in the last 3 years too. I am not as familiar with the U.S. numbers, but I can't imagine them much different. So, grads may well be turning the car around because, from a distance, it looks like the store is probably closed.
I'm not so concerned about minimal interest, as long as those that are interested are the sorts of people who are a great fit for the job. Remember, it doesn't take that many new recruits to backfill the outflow, and not all federal jobs require a university degree.