Because ZDI negotiate. As a bug bounty participant in the official programs, you aren't allowed to negotiate.
ZDI, on the other hand can say: "We want $10M for this iOS zero day, or we don't report it to you." And the process of negotiation goes back and forth, but the end result is, Apple will pay considerably more to ZDI than through the direct program.
ZDI, on the other hand can say: "We want $10M for this iOS zero day, or we don't report it to you." And the process of negotiation goes back and forth, but the end result is, Apple will pay considerably more to ZDI than through the direct program.