Because he's a muppet.That part irritated me a great deal. It seemed very arbitrary on Jackson's part. He weakened the Riders immensely and then compensated by sending them Elves. And yet, why would he have weakened the Riders in the first place?
This is sufficiently evidenced by his handling of Grima. They spent all that screen time painstakingly building up Grima, showing his motivations, and his influence over Theoden, and then completely abandoned it for No fucking reason and replaced it with 'oh but saruman was posessing him all along look'.
The big problem isn't even the departure from the original story (again, like the elves at helm's deep, for no readily explainable reason, as sticking to it exact would have made for a more powerful scene) it's that they WASTED ALL THE FUCKING EXPOSITION! build up the situation, and then completely ignore it! Way to go Pete, great storytelling there!