Before the Puncture, the wizards and sorcerers of Multiverse A drew their magic from the four Elemental Planes. (Krynn’s wizards used the power of the three moons as a focus.) But the rearrangement of the planes after the Puncture – and in particular, the fusion of the Elemental Planes into the Elemental Chaos – meant that mages lost track of their power source and were left unable to use magic at all. This multiverse-wide calamity, called the Drought, crippled the ability of many mortal civilizations to defend themselves from extraplanar invaders.
Also contributing to the Drought were systematic differences in the old and new “rules” for spellcasting. The manner in which eldritch energy could be shaped had changed, rendering spell components obsolete but requiring a subtly different sort of mental discipline to master. A powerful wizard with a flexible mind and a good teacher can relearn his craft in a day or two. Acolytes take longer. Relearning the craft without a teacher approximates the impossible.