We don't really know why the Bronze Age collapsed.

The Bronze Age loosely took place in the last years before the common era (CE or AD). The term “Bronze Age” is applied to the varying lengths of time (by area) where copper and bronze were the chief hard materials in use. 

For each area where the Bronze Age refers to, the Bronze Age ended 1200 or fewer years before the common era, where it was succeeded by the use of Iron in the Iron Age. During the early years of the Iron Age and the late years of the Bronze Age, in the Aegean Region, Southwestern Asia, and Eastern Meditteranean, an event known as the Bronze Age collapse occurred. 

It was an event generally believed to be violent, sudden, and culturally disruptive. There were many changes in rule, economy, and culture around 1175 BC (or BCE). During this period, the Greek Dark Age, almost every city between Troy and Gaza was violently destroyed and abandoned and almost no records were preserved. 

These cities included Hattusa, Mycenae, and Ugarit (all of which are today preserved as archaeological sites). There is a flurry of speculation about what exactly happened. Theories range from volcanoes to earthquakes, from drought to migration, from raids to warfare changes. 

It’s definitely worth reading up on, as it’s one of the greater historical mysteries in the last few thousand years.

(Source)