Because the Klingons were watching everything he did as he did it, so during the few seconds of the shields deflecting their shots they finish adapting their weapons and the cycle starts over.
Nitpick: Worf could have done it from the Tactical console. He's even done it before. IIRC Data has done it from the Ops console as well.
Excellent point, if Worf and Data werent busy and Geordi was looking at any panel but shield control the battle would have gone much different. Ah the demands of the plot...
They had no idea HOW the Klingons were shooting through thier shields (the audience may know all, but the characters don't). If they hadn't been taking so much damage, someone might have run a scan and noticed the matched frequencies, but that's the fog of war. As it stands, Worf and Riker were more interested in shooting back than fixing the shields.