#1 cannot get all clearances until 2 yrs of age
#2 Males are a dime a dozen. Yeah there are special ones, but there are a plethora of stud dogs out there to choose from. Until they get infirm, generally fertility in males stays quite constant throughout their life. There is no physiological advantage to breeding to a young male. Now, why would you bank on the 2 yr old who's never been bred, when you can choose an older guy with a proven track record of producing nice puppies, puppies with clearances, puppies with titles, and oh yeah -- the fact that he's still alive at 5, 7, 9, 11 years old is a BIG advantage over that two year old guy.
Not saying 2 year old males should never be bred. They gotta start somewhere. In my opinion they should be used on either #1 their owner's own proven bitch or #2 by an experienced breeder with great knowledge of her lines.