I have a small theory of my own about this. I think the only reason Link can charge up a Skyward Strike in SS is because thats the Goddesses way of helping him. Think about it. Link is the first chosen hero chosen by the Goddess. It would only make sense if she helped him a little. In the game the Goddess sends a few things down to the surface in order to help Link- Goddess Cubes, Goddess Walls and the Bird Emblems ( Found in Spirit Springs, The Statue of the goddess and in the sword purifying rooms.) Those things are there because the Goddess put them there for her chosen hero and not for anyone else. With Link possessing the Goddess Sword he is the only one who can access them. I don't really agree with the sword beam in other games being the same as the Skyward Strike idea because they are two different things. Skyward Strikes requires the Goddesses power and for Link to raise his sword skyward and the sword beams in other games do not. More proof is that in the original Legend of Zelda and in Legend of Zelda Adventure of Link, Link doesn't even use the Master Sword and he has a sword beam. Sure, he has a sword beam in ALTTP but I'm pretty sure thats just a thing Nintendo did with the older Zelda titles. They gave Link a sword beam with no intent that Skyward Sword Link would also have a beam of sorts. Nintendo didn't realize, too, that when they made the Master Sword it would later on use a Skyward Strike.