Cline has argued for systems collapse. i.e. It wasn't any one thing that caused the collapse, but several things in conjunction. Others argue differently. e.g. James Lacey, who comes at things from a perspective of military strategy and economics, has argued that the systems collapse model really underestimates how much war can accomplish all on its own. He's of the opinion that, whoever the sea peoples were, they caused the collapse.

This article doesn't really wade into that dispute, but instead focuses on why new empires didn't rise once old ones collapsed. Iron smelting originated before the collapse, but didn't really spread at first. Bronze age iron was probably really awful stuff, and soldiers tended to be conservative when it came to technology. They wanted weapons they could trust, and that was copper or bronze. As long as the great empires persisted and the trade of tin and copper continued, nobody was forced to use Iron, so nobody was motivated to get good at producing quality iron weapons.

Then whatever caused the collapse happened, and the ingredients for making bronze became difficult or impossible to get. It was iron or back to copper and stone. So, peopled adopted Iron and that gave smiths the chance to improve iron smelting techniques. During the bronze age, a would-be empire just had to get their hands on a decent amount of bronze and they could steamroll everybody they came across because they all had copper or stone weapons. After iron technology spread, it was iron vs iron. Would be empires got hung up fighting their well-armed neighbours and the snowball couldn't get rolling. The few empires that were left, such as Egypt, contracted. They no longer had a technological advantage once those they formerly ruled adopted iron.