I would think (for most games) it's to make sure the input is different from your normals. A quarter circle is a good example - say my opponent jabs with an attack that hits high, then crouch-blocks anticipating a reaction; I may want to crouch under the jab (press down), then do an overhead attack (forward+[some button])that will hit the crouch-blocking opponent. A quarter circle (down, d+f, forward+[button]) will be distinct from this, so I'll only pull it out when being as explicit.
You don't want to do specials accidentally as well. If specials were a case of one direction and an attack button, at worst for a masher, they'd pull it out very quickly and most likely get punished, an experienced player would pull it with a slight mistiming (was holding forward to move, didn't release before attempting a normal). In the general scheme of canceling, light normals are canceled by heavy normals, which are canceled by specials, which are canceled by supers, and anything former is canceled anything later on the line. When you're not ready to do a super, a blocked of whiffed special will leave you open for punishment.
All said, I have come across some ridiculous inputs for specials. I recall Chaos Code having the input for a move being quarter-circle backward, half-circle forward + kick, and the game was quite asinine about getting all 7 directions in the right order, with no interruptions, and in good time. On the other hand, I find Skullgirls quite elegant in its inputs for specials and supers: a quarter circle or dragon punch will start either, with one attack button for a special, and two for a super. It's even become lenient on 360 motions for super-throws, accepting forward, down, back, up.