Because the attendees are high-value targets who often have elevated permissions inside the firms or governments they work in, and that's worth even more.
On a separate note, Zerodium is dead now. They're in the middle of an active fire sale, but the Zero Day market's bottom fell out now that countries are increasingly moving exploit development in-house or to vendors that can do both zero day acquisition AND exploit deployment (which Zerodium cannot do as an American company).
