Because they didn't want to open the hatch and drown? Because they couldn't see any way out of the situation? Because they were in agony with blood coming out of their ears? Because they were obeying orders, man your post?
There was a story handed down that Lt. Dixon was carrying a 20$ gold piece given to him by his fiancé a few years earlier, for good luck. During an engagement with Union soldiers he was struck by a bullet, or musket ball. That 20$ gold piece saved his life, it stopped the projectile from hitting him. When they opened up the Hunley one of the researchers stuck her hand into the muck inside the sub, pulled it out, and she was holding that gold Double Eagle in her hand. True story. It’s documented here: https://www.hunley.org/artifacts/