Why modern warships do not regain heavy armour

Why did navies not add much more armour after the Falklands War?

Because practical cruiser or frigate armour cannot reliably stop modern anti-ship missiles without an unacceptable weight penalty.

The Falklands experience does not lead directly back to battleship protection. Modern missiles combine large warheads and high impact energy, making meaningful external armour prohibitively heavy for ordinary surface combatants. Splinter protection and internal survivability remain useful, but they solve a different problem. Designers instead use thicker hull steel and local protection around magazines, improving resilience without pretending the whole ship can defeat a missile through armour alone.