Examine what happens to a cow during her re-breeding stage.
She needs feed, enough to meet her other needs with enough left over for re-breeding to occur.
Young cows specifically two year olds, are still growing, nursing a calf and trying to re-breed.
Translate that to bulls. Yearling and 2 year old bulls are still growing and trying to breed cows. They need enough body reserve to get through the season because their thoughts generally don't include much eating.
Too much body condition, they lose athletic ability, plus the fat cover can insulate the testicles and damage the semen quality.
Too much protien in a bull will basically be passed out the other end with no benefit. Too much energy can damage their feet and livers. A balanced diet that bulks them up a bit before breeding is a good idea. Trace minerals like zinc are very important to sperm production.
A forage only young bull isn't very attractive, unless that forage is grain silage (then forage only is pretty misleading) and dairy quality alfalfa.
Instead of a generic my "bulls were fed a high roughage diet" breeders should be willing to share the exact diet. I have provided feed analysis but it usually just confused some buyers. I feed 8 pounds of grain and free choice 50/50 alfalfa grass hay to my bull calves. They gain slower than a pushed bull, but last a lot longer. The same ration if ground and mixed together would result in another .5 lb per day gain.