I am trying to recall that day, and I think the barrier, the gate, was locked. It should have been unlocked when you came up. Prior to that, and during the overnight hours and early morning, people were being referred mostly to Bank and Wellington, so it was an oversight, if I recall the date correctly, that the gate was not open, keeping you on the other side and preventing you from coming up. It's one of those things that shouldn't have happened, so it's a lesson learned.
At the same time, there was a lot going on on that particular day, and people were multi-tasked, so I can understand the oversight. However, I am also quite understanding of the anxiety one would feel at arriving at a locked gate, when one had to get into the chamber for a debate.