The people were told to focus very deeply on a certain aspect of the scene. Maintaining that focus means explicitly blocking things not related to that focus. Also, there is social pressure at the end to have peformed well at the task; evaluating them on a task which is intentionally completely different than the one explicitly given is going to bias people away from reporting gorillas.
And also, "notice anything unusual" is a pretty vague prompt. No-one in the video thought the gorillas were unusual, so if the PEOPLE IN THE SCENE thought gorillas were normal, why would I think they were strange? Look at any TV show, they are all full of things which are pretty crazy unusual in normal life, yet not unusual in terms of the plot.
Why would you think the gorillas were unusual?