Probably pressure from other top players. I mean if they do it in Marvel then they do it in other games as well and not only that but they play together in teams as well (Justin and Chris won teams as well). It's just safe consistent money provided you make top 8 at least.
While it's economically more feasible for Chris to not split in Marvel, he makes that up by getting pot split in other games he gets top 8 in. It's always beneficial for J Wong considering how many games he places in. I don't know if they split with Xian though, they might have.