I supose that you can look at it as a cylinder is the easiest shape to roll - thus log rolling. Once you add a hump, it's like having a branch sticking out of that log. It'll push water. A wide, flat boat should have the best initial stability, but that stability works upside down as well, making it harder to roll.

: Higher volume makes the boat float higher while on its side which tends to
: make a "hump" you need to get over to roll up. Flattening the
: deck does make the boat more stable when upsidedown, but the hardest part
: of rolling is when the boat is at about 90 degrees. With the lower deck,
: the boat can sink down farther making it easier to get it under you.

: Lower volume boats tend to be easier to roll, however it comes at the cost of
: lower secondary stability. It is the reduced secondary stability that
: makes the boat easier to roll through the transition.