Cargo flying is a lot easier to outsource. As long as it gets there on time in one piece the customer doesn't care who flew it or on what terms, price is the main factor.
Mainline pilots generally won't care about not flying the worn out freighters, as long as they get to keep the shiny new passenger planes.
Management find it a lot easier to subcontract cargo flying out. If they bought in another operator on passenger routes there would be regulatory issues, instant union opposition and passenger complaints about not getting what they paid for.
A couple of cargo planes chartered in, just for the peak season of course, won't bring much opposition and can easily become more in number and permanent.