Explosives, gunshots, etc. are outside the normal sensory experience of most people and they are not accurately perceived. Even stimuli such as fire alarms which are intended to produce action frequently leave people standing slack-jawed wondering what to do.
For instance, when under attack from long-range gunfire, it frequently takes multiple shots, possibly even seeing multiple people get hit, before people understand what is going on. In eyewitness accounts people often disagree about which direction the shots came from, how many shots there were, etc. In fact, sometimes people get shot and don't know that they were shot until later.
A person getting injured by a small bomb in a theatre might understand they were injured, and not know how. Maybe they got shot, maybe they got stabbed, ... Everybody was looking at the screen so it is not clear. That person might go out to get help and people will focus on treating the injuries rather than understanding the situation.
It's actually pretty hard to make homemade bombs that reliably kill. It took the Unabomber more than a decade to figure it out, and that is why they have terrorist training camps. It is not very dangerous to make small pyrotechnics, simply because they don't contain much energy. If you were screwing around with those cough lozenge detonators and were making bombs big enough to be really hazardous, you would probably blow yourself up before you blew up anybody else.