Okay. Let's make a thought experiment out of this. Let's make special moves in SF4 single-button maneuvers. What changes?
Now, the player has access (on Ryu, because he is The Example Character) to dragon punch, fireball, swirlykick, super, and ultra, with a single buttonpress. Now everyone can do any move they want when they really want to, as long as they remember which button does what! For the sake of simplicity, let's assume everyone can always remember the right button.
Immediately people get into serious trouble. If a fireball is so easy to do, people will do it. A lot. They will win a little bit more at the start, maybe, but they will quickly get body organs blasted off them once people know to expect fireballs. Why? Because special moves take a long time. If someone jumps your fireball, or blocks your dragon punch, they get a free full combo, even if they were at half-screen (or greater!) distance when you started your special move. A simple, single-button thing can get you into incredible, super hyper crazy amounts of trouble. What we can learn from this is that special moves take more effort to do because doing them indiscriminately can be catastrophic.
Let's go one step further and say your typical 'normal' moves are special inputs. Quarter circle forward + swirlykick to get a standing Roundhouse, dragon punch + fireball to get a crouching jab. You're probably expecting what happens next: your opponent jumps in, a clear opportunity for a quick punish with a crouching Fierce, but you screwed up the motion and now you're doing a basic fireball, which is the perfect thing to NOT do when your opponent is jumping in. You end up in an incredibly vulnerable position for the better part of a full second because you couldn't reliably do the motion.
By contrast, when you screw up a motion in an actual fighting game (where the specials are not single buttons)--you get a quick, low-investment low-risk normal move. That dragon punch came out as a jab and you have a whopping three frames of vulnerability. Sure, you didn't dragon punch him like you wanted, but you're also not having to eat a vast amount of reprisal. So from this we learn that special moves require big inputs to ensure your safety.
Let's also not forget the variability in special moves. If you do a, say, Falcon Kick in a Smash Bros. game, you know exactly how far it's going to travel, at what speed, every time. Ryu can throw a fireball at three different speeds. Gouken can throw fireballs at three different trajectories! All because they can use the same motion with a different-strength button to get a different tweak on the move. So special moves use motion commands so that the buttons can give them varying utility.
(Not that the Smash Bros. games are poorer for this, by the way. Smash has the interesting aspect of, among other things, only having one hit 'height'--either a move connects or it doesn't, there's no "high" and "low" and "mid" moves to worry about. There's a lot more too, but my point is it's a different world there, and the system reflects as well as creates that.)
Let's look at one more thing. When you do a half-circle back move, and your opponent wants to hit you at the same time, you might actually be okay because the command involves the same motions that also get interpreted as blocking. Executing the move is safer, in a real way. A dragon punch has no such luxuries, all its moves are forward or down. Actually, it was a really big thing when people learned the input shortcuts for dragon punch, as down-forward, down-forward--that's really, REALLY big because it means you never have to stop crouching even while inputting the DP. That means you can hold a down-to-up charge move while you DP. Or continue to crouch below moves that only hit high. That is a massive, super huge thing--a move's safety is built right into the commands needed to perform it, and each move's commands are decided by the designers on purpose with these effects in mind. You're not *supposed* to be able to do a full-circle command without having to buffer it out of something else. Without having the command inputs the way they are, a lot of depth would be lost.
If you don't believe me, pick up a copy of Capcom Vs. SNK 2: Match of the Millenium, for Gamecube (and I think the PS2 version does this as well). Start to learn the game in GC-ism--all your specials are on different directional inputs of the right analog stick--and I don't mean sweeping quarter-circle stuff, I mean like Raging Demon is at precisely 45 degrees up and toward, and a fireball is at straight forward, and dragon punch is at 90 degrees up; click the stick in that direction and out comes the move. Take careful note of what happens. And then tell us about your experience.