He needed to gain the Borg Queen's trust. If he didn't have her trust him, she would have noticed him walk up to the coolant tanks and would have had a borg drone stop him. The Borg Queen was distracted by the quantum torpedoes that Data launched at the Phoenix.
The above answer is implausible, since Data is at least as strong as the borg drones, and probably faster. He almost certainly could've made a break for it and ruptured a coolant talk before anyone could have stopped him. @#!*% , he could've just detached his arm and chucked it at one of the tanks. There's probably no good in-universe explanation for this one; the producers wanted to create suspense and faking Data's defection was a good way to do it.
He made a run for it earlier, but got stopped by a force field and surrounded by seven or so drones. The Borg queen went from happy (in a creepy sort of Borg way) to confused to about to throw a tantrum in a matter of seconds, so that provided Data with all the distraction he needed to not get stopped by a force field.