Battlecruisers were essentially a compromise design, trading either armour (in the British case) or guns and armour to a lesser degree (in the German case) for speed, at a time when a fast, fully armoured battleship carrying the largest guns was not feasible. The idea, at least on the British side, was that speed would compensate for armour, making the ship harder to hit and able to make a quick escape from a squadron of slower battleships.

By WWII you could make something like the US Iowa class, fast, well-armoured and armed, but by then battleships/cruisers had become vulnerable to carrier-borne aircraft (as illustrated by the sinking of the Prince of Wales in the South China Sea in December 41).