Apparently it was a contractual issue and that his song "Perfect Day" was not cleared in time for her to perform in America as it was in the UK.
She also didn't have another song prepared for the show making her unable to perform.
Reps for Susan released a statement:
"Susan was due to perform a chosen song from her new album The Gift on America's Got Talent in Los Angeles. It is with real disappointment and regret that she has been unable to do so due to unforeseen circumstances relating to the song.
"Susan is very disappointed as she had been looking forward to it immensely. She is now on a plane heading back to London with a heavy heart. Susan really looks forward to being able to perform her new songs very soon."
Chin up, girl!
[Image via WENN.]