You are forgetting that even though both were running behind the same line Brown had 5 years to get the playbook and timing down so that all he had to do was react and not think. That makes a big difference when you have to think about every step and react. Brown @ 5 years vs Richardson @ a few weeks is like comparing green apples to ripe apples. Green is pretty sour until you give it time to mature.
Eh, he had only a year and a half advantage with the new coaching staff coming in back in 2012. Really, the main culprit is Trent's vision that seemingly makes it look like he can't find the open gap to save his life now.