I think it's mostly that you get a lot more mechanical advantage when tightly rolling than stacking flat, which allows a lot more air to be squeezed out. Once rolled, it's also easier to prevent it expanding: you just have to stop it unrolling, not by applying constant pressure to the whole surface. This is easier in smaller bags like backpacks actually, as compared to big hardshell cases, as it keeps the rolls fixed in place.

What gets tighter: wrapping tape around something or laying it along something?