
I'd imagine giant breeds are not normally considered due to the shorter lifespans, why spend all that time and money training an assistance dog if it's working life is expected to be shorter than average? Also their higher mantance costs for the owner
Then some breeds could also be out if they would not suite an average pet home, something with a very high working drive most pet homes probably couldn't cater for its needs.
Then you have some breeds are just more biddable and tolerant of certain things than others, something you would want in an assistance dogs espicaly in one for a child.