Rebecca has said the same thing though, that Randall was just easier, calmer. Jack also told Kevin that he's the big boy he didn't think he needed to worry about when they were at the pool. I think Jack and Rebecca used this as a device to motivate the kids to behave. Unfortunately in a child that imprints like Randall this was detrimental psychologically.
I wish I could care more about who's in Kevin's bed.