It could be to do with the university being known for heavily enrolling many students abroad who may wish to practice law elsewhere. This is because many overseas students study in London.
Maybe it's to do with the candidates on any given year just aren't impressive enough. This can change all the time as it is all down to the person and how well they are percieved.
I can tell you now, KCL is consistently ranked in the top 10 law schools in the UK and no employers in the magic circle is going to shun you away just because you attended that university. It's all down to you and the way you sell yourself. I honestly believe that a student studying an LLB at the worst uni in the UK could still land a job in the magic circle but I also agree that they would have to be spectacular and their non-study achievements and work placements would have to be great too. but it is possible.