Not to mention, like a lot of Frank's characters, the voice was basically a variation of Bert, which makes inserts with his Lefty feel like Bert is getting back at Ernie for all the times he's driven him crazy. Caroll's voice, while not perfect, sounded more convincing as a shifty shyster. Plus, without the Jim/Frank chemistry, there's more of a genuine awkwardness between the unsuspecting Ernie and the scheming Lefty.