Perhaps it is because getting sent left or right has a greater window for the player (not necessarily Snake) to DI. The goal of DI is usually to influence your character in a direction perpendicular to the direction you are sent. That's why most people who DI correctly survive when flying towards the "corners" of a stage, so-to-speak, because that is the greatest distance from the center.
So maybe since utilt sends Snake closer to a 90 degree angle, it will take a longer time for the character to move to the "corner area" when attempting to DI down+towards stage.
However, an attack to the sides, say fsmash, doesn't send Snake as close to a 180 degree angle, in fact, it often sends Snake at an angle close to the direction towards a "corner". Thus the DI (in this case it would be up+towards stage)is more effective since you reach the corner quicker.
Just my theory and I'm not sure if I worded it well enough to make sense.
