How do I explain my rejection?

I completed an Easter vacation scheme with a top 20 law firm earlier this year but unfortunately did not manage to secure a training contract on the back of it.

The feedback I got was very positive but I was told that I was a little bit too quiet compared to the other students. I am therefore going to start applying for training contracts with other firm but am worried that they might ask why I failed to get a job as a result of my vacation scheme. Any suggestions as to how I should deal with this?