Most simply put, the idea is that a chassis with less flex gives a car a more stable baseline on which its other components can be built. The stiff chassis eliminates one variable in the equation of what makes a car go faster and handle better, making it easier for crew chiefs and drivers to work on other factors. Some teams began to reduce that in hopes of saving weight, using a stronger metal alloy to make up for any loss in stiffness. It is very tempting to stack up a series of events and let them lead to what seems like a logical conclusion. All of those incidents have happened since the rule about thicker chassis tubing has been in place.
Some people think that answer is obvious.
They know what's killing the drivers. They're like bricks, and when the car hits the wall it does a lot of bad things. It may seem that simply changing the thickness of the metal or ordering the removal of a few bracing bars would solve the problem. Perhaps, though, doing so willy-nilly would create additional problems. No team adding bars to help a car's handling did so with the intention of making the cockpit more hazardous to the driver.