The payouts are based on what their 'clients' are willing to pay in turn for the exploits. There's just less of a market for Linux kernel exploits. If nation-state actors are involved in deep APT style attacks where they would leverage low level kernel exploits they are going to either develop the exploits themselves or acquire them through their own clandestine channels. Purchasing that stuff from a publicly facing company that could potentially be compromised themselves is high risk and leaves too obvious of a trail.