Can you imagine a bomb in a crowded audience only injuring four people today?
And yet in the 1950s they didn't.
What was different about the mindset of someone from the 50s? Were they inured to explosions coming out of WW2? Were "pranks" with pipe bombs a normal thing? Was it just that nobody had ever been killed by one so they didn't recognize the threat? Was the 50s mind somehow more stoic?
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.
It was also evident in how free-er kids would walk around -- a 10 year old having adventures around NYC by itself instead of being taken everywhere by their chauffeur parents.
Additionally, the low explosive gunpowder used in these bombs is considerably less powerful than what you'd encounter in a modern day terrorist attack using high explosives (e.g. TATP, PETN or RDX). Even compared to other deflagrating low explosives, regular gunpowder is pretty mild compared to e.g. flash powder[1].