If you mean dub Osborne, it's mostly because he doesn't do much in 1. His voice actor shines when he's giving speeches and trying to be charismatic, and he shows that off most at the end of the game.
I quite disagree. He isn't bad, most certainly, but he doesn't match up to the legend. It's like what happened with Xehanort in Kingdom Hearts, The new actors they got for him aren't necessarily bad, but when put up against Nimoy, they will ALWAYS pale. The voice change made him less charismatic. A large reason why Osborne is so charismatic in the other games is his VOICE, he just sells every line of dialogue and it makes you think that maybe he has a point. That isn't there as much in 4.