I kinda touched on this matter in my own thread
http://forum.barrowdowns.com/showthr...hlight=Saruman
I think the confusion is understandable - Gandalf the Grey, an inferior power to Saruman the White, had an epic struggle with a Balrog. Fighting the Ent's forces seems like nothign in comparison.
But Inziladun supplied a great quote that may hold the answer:
'I don't know what Saruman thought was happening; but anyway he did not know how to deal with it. His wizardry may have been falling off lately, of course; but anyway I think he has not got much grit, not much plain courage alone in a tight place without a lot of slaves and machines and things, if you know what I mean. Very different from old Gandalf.'
Saruman is perhaps just not suited for combat, lacking the necessary courage or fortitude to expend himself and his essence in a life-threatening manner like Gandalf did.