1) Location. While London is definitely business and cultural center of Britain, I believe it's a bit of a slog for students to travel down to the recruitments fair, especially since most of the best universities are located outside of the city. SO going to them would definitely help.
2) Most of the companies pitching are relatively unknown to the general masses. My best hires are people who either approached us because they like why we're doing, or people in my professional circles. Professional developers fit nicely into these two buckets, were most students don't. Great companies like Tuxebo and Conversocial don't really have mass appeal. Subsequently, it's hard to get a student who doesn't truly understand the company/space excited about being a part of the team.
The second problem is even further exacerbated as early startups don't want "employees". They want "Employee #1"; someone who is passionate and ready to help push the company forward.
It's a tricky problem.