Could be any number of reasons. Preference, free factory ride, etc...
I think it depends ... most of them could ride anything they wanted, really. They sometimes pick and choose, depending on type of race. I know Haaker does his enduro cross races on 4T, because he likes the torque delivery better for the obstacles. But for Erzberg he took 300 2T, like everyone else in the top ... They all prefer 2T for redbull tough series ... I think it has very little to do with "free ride", since they are all factory riders. Not just factory 2T or 4T riders .... the company will give anything they want, as long as they win ... LOL