The designers of 4E didn't like disarm.
Disarm is too small an effect to show up in 4E really. People want disarm to be some big huge effect that permanently alters combat, but 4E combat is modeled after anime or WWE "wrestling" where someone is bleeding on the canvas, they take a healing surge, spring back up and are suddenly jumping from the top ropes slamming their opponent to the mat. What happens when The Undertaker has the chair knocked out of his hands or Jackie Chan's vase gets shattered? They pick it or something else up and keep going.