I think the explanation for it is quite simple. Both White Noise and Brogue Kick are a lot easier to pull off on large wrestlers than the Celtic Cross (Razor's Edge).
As strong as Sheamus is, he's not going to be able to shoulder press large guys into that move. The only way he can Razor's Edge those guys is if they get on the ropes backwards and he sneaks in to slap it on them without having to shoulder press them.
Otherwise, you may occaisionally run into a Ryback/Tensai sandbag situation that just looks awkward. This way, Sheamus can trample any opponent including Big Show without missing a beat. Brogue Kick can come out of nowhere too, which makes it more exciting for fans (ala Super Kick, RKO, Rock Bottom, Stunner, etc)
Originally Posted by Asenath
It's a sword from Icewind Dale - a paladin's sword, +7 against evil. Which means it would be a perfectly named move for a heroic figure. And when they let him, you have to admit Sheamus cuts quite a heroic figure.
Does that mean Sheamus is an RPGer? I didn't know that about him! Sheamus gets some major cool points in my book if that's true.