Bloody good question. A walkround is nothing short of the most essential pre-flight check, and the control locks are part that check. (on most a/c)
Even pre-start and then at the power checks you would check the control surfaces for full and free movement. This obviously was never done. I can only assume that a serious case of "done it a thousand times-itus" crept in.
An very expensive way to learn a very serious lesson.
Regards.
