There are two ways to look at it........
1 If they choose one of their own ATSAs to go forward for training, he/she will have undergone some sort of selection (in the past, a pre-NATS Brizzel used NATS style selection testing) and they will have a good idea as to the candidate's suitability for the job and whether the candidate will get through a course. You on the other hand, they don't know anything about, you are an unknown quantity.
2 You've already taken and passed an approved course, and as I've always said give me someone who's passed a course and I'll give you a valid ATCO.
And before anyone starts harping on, I've never been wrong yet.