beause im blessed with good genetics for biceps and i know that the second i train em heavy they will grow disporportionate to my triceps.. and i hate being unbalanced... and i prefer larger triceps to biceps anyways... so i train my triceps like crazy and just do a little for my biceps..after about a year of doing this, with the occasional heavy bicep day, now my arms are getting proportional.. i still have good biceps though..
plus another thing i notice is that when i trained my biceps hard, they would try to help during my back workouts. now that i leave them alone, they dont try to contract during back movements, which allows me to put more weight on the lats, and they have ben growing better because of it.