They (permanent forts, intended for shooting back at the enemy from, as opposed to other fortifications such as temporary bunkers for fighting and permanent fortifications to protect things but not designed for shooting back from (e.g., bunkers to protect command centres)) didn't quite stop being used, but they become much less important.

WW2 showed that they could be destroyed and/or bypassed. After that, they didn't appear to be very cost-effective. WW2 weapons used included shaped-charge explosives designed to penetrate the thickness of reinforced concrete used in such forts (and thought impervious to regular explosives), and bombs of unprecedented size designed to penetrate hard targets. Forts could be bypassed by airborne assault (or captured by airborne assault, such as was the fate of Fort Eben-Emael which fell to a glider-borne attack in 1940 (which used shaped-charge explosives to knock out guns, etc.)). Harbour defence forts lost some relevance when aircraft carrier fleets could stay out of range and bomb the harbour.

Since fortifications remained in use, such as well-protected deep bunkers, weapons useful against forts (but more strictly developed as bunker-busters) continued to be developed, and discouraged further building of forts. This further development includes nuclear weapons. Weapons such as these can make concentrating one's defensive force in a single place risky - all the eggs in one basket potentially resulting in a basketful of broken eggs.

Another major problem with large forts is that their location is usually known well in advance, and invasion plans can be made to bypass them, or an attack on the fortress can be prepared (as in the case of Eben-Emael). This means that there is a good chance that they will prove ineffective in a war, and fail to justify the cost of their construction and maintenance. Much weaker fortifications (such as quickly-built bunkers) can be more cost-effective.

Both of the above encourage the use of smaller, cheaper, and more dispersed fortifications.

In many ways, the fate of large forts after WW2 parallels the fate of battleships - too expensive and too vulnerable. Just as smaller warships stayed in use, so too did smaller fortifications.

Forts have made somewhat of a comeback in modern counter-insurgency warfare, where the main threat is sniping or mortar attack, both made more difficult by even a small and primitive fort. Example include fortified bases during the Vietnam War and the use of brick and earth forts in Afghanistan and other places with current fighting.