Because they have an agreement for a higher flow

Also, they have the largest Pilot group.

Now...as to why we don't have double Piedmont's flow when we have twice as many pilots remains a mystery (aside from the contractual rate in the contract). As I remember it the pilot group is about a 3:2:1 ratio for Envoy, PSA, and Piedmont respectively, but the flow is 3:1:1, for now.