Thats because our bicep genetics suck. I cant get the split on the bicep that divides the 2 heads, even at a very low bf%. If you notice a lot of guys have that problem, not everyone is gifted with those nice defined rocks.
I agree that genetics will dictate the biceps peak. However, the exercise that hits my peaks the best for me is cable curls, with extended contraction with cable handles right at my ears. I find that a lower weight allows me to hold the weight longer at the peak bicep contraction, and really helps me focus on the target muscle and squeeze.