I prefer to think of the lockout mechanism as a player engagement booster. After all, why would a player pay attention to an event and get bored if they were to clear the whole thing out long before the end is due? I've heard of legends and old/distant rumors that people who finished everything on day [{End}-X] became dissatisfied, which led to complaints about how things got boring and end up leaving for a while (or at least up until the end of the event). If I searched the forum for some of those messages, I'm sure I'd find some. Not feeling up to that, though.
On a more cynical note, it's probably also a sales booster. People think less of spending chunks of change when the sales are spread out "far enough" (with the seller being the one most predisposed to knowing/learning how "far" "far enough" is), and so it makes sense to spread sales across the event. That plan ends up taking a nasty drubbing if people get bored enough to quit paying attention in the middle because they're done with everything the event can offer (for free).