Because if you use a flat percentage that applies to everything and is designed to dampen your 60 or 120 second bursts, then your basic combo is going to end up being trash compared to anyone who isn't synced. It also may inadvertently target people who are just barely over level and don't have the abilities to make up for it. I just think traits for every 10 levels will also be better for future-proofing further expansion drops.