Saruman makes no effort to defend Isengard
If we examine the scene of the Entish invasion of Isengard, we see that Saruman actually does nothing to muster any defense. He is shown several times just watching the disaster from a balcony on Orthanc.
This may be because he realizes the situation is hopeless
Saruman's army is away at Fort Eorlingas, with only a token crew of Orcs left behind at Isengard. They are hopeless to repel the Ents. Saruman cares nothing for his troops and makes no attempt to lead, organise, or defend them.
The Orc strategy of using fire on the Ents was nullified
We do see an Ent on fire during the battle sequence, but the fires are quenched and the Ent saved when the Ents break the dam.
TL;DR Saruman locks himself in Orthanc with a view to talking his way out later
One of Saruman's greatest weapons is the sorcery of his persuasive voice and his mastery of diplomacy. Any aggression against the Ents makes this less likely to succeed (though he does of course attack Gandalf the White once provoked).