The fill rate is not an arbitrary rule. The goal of ELP is to allow more orders to be filled on Direct Edge (i.e., not routed to INET/ARCA), thus saving Direct Edge customers routing fees (and making DE more money).
If your ELP members aren't filling orders, then customers pay more to have their orders routed and DE makes less money.
I think the point about saving DE customers routing fees is the most important one here. If it weren't for DE's advantages (pricing, speed, lower fees) nobody would use it.
If your ELP members aren't filling orders, then customers pay more to have their orders routed and DE makes less money.