two factors.

One is pressure. With no leaks in or out of the cooling system, if a cap were blocked, there is a limit to the pressure the system could develop. It might be enough to blow a gasket or a hose, but it would probably be less than 30-45 psi, but with very little volume potential(until the pressure were relieved by something bursting and the coolant starts boiling since the temp is possibly above the one atmospheric boiling point with a hot engine*).

With a head or head gasket leaking compression pressure into the cooling system, you have not only the potential of 100+psi in the system, but the potential (depending uon the size of the air leak) almost unlimited volume. This volume could easily override the ability of even a good cap to vent it to a safe level.

Bob
Pressure increases the boiling point temperature, as does the addition of the anti-freeze material.