Why didn't Hashirama break free from Edo Tensei in the Chuunnin exams?
You will note that Tobirama, in the same scene tells Orochimaru that achieving better fidelity/accuracy/efficacy with edo tensei will be the cause of his downfall, since this time they have been revived with nearly their full power.

This strongly suggests that Orochimaru did not perfect edo tensei at the time of the Chunnin exams and that Hashirama was not at full strength and capacity.

Which explains why he did undo the binding at the time of the Chunnin exams - he couldn't.

You will also note that Hashirama doesn't seem to recall Orochimaru, further suggesting that he was not fully revived at the time, though that could simply be his personality...