Why use the Canon 1DX? The photographer has a long-standing and public relationship with Canon. Why not use the Sony A7S? The photographer uses a Canon 200-4

Why use the Canon 1DX? The photographer has a long-standing and public relationship with Canon. Why not use the Sony A7S? The photographer uses a Canon 200-400mm f/4 lens, and the nearest Sony equivalent is 70-400mm f/4-5.6, which means at the long end it captures only half as much light, instantly negating any possible ISO range advantage offered by the A7S.