[The Saucepan Man]

I doubt that Jackson and co really thought about it that much. They just wanted a nice effect that (non-book reading) viewers would like and, at the same time, would show that Gandalf's powers were now superior to those of Saruman.

Hey, you could come up with any number of theories. Perhaps the restrictions placed on the Istari prevented them using such powerful magic on "lesser" beings.

Although it seems to me that Saruman would have been hard pressed to take all of the Ents out and, Lolidir pointed out, a fireball would have been practically useless once the Ents undammed the river.