The Bronze Age collapsed for many reasons but the event that really kicked things off was the explosion of Santorini. The empires that died were not quickly replaced because after extensive famines and invasions they were replaced by different people with different social objectives.
The same thing happened later in Europe when Germanic and Asian tribes displaced the Roman and Celtic tribes of Western Europe.
This is only challenging to understand if you falsely believe civilization and technological advancement to be socially preferable. Historically that is not the case. Civilization tends to emerge not of preference but out of unwanted necessity to regulate resource production/distribution.
The same thing happened later in Europe when Germanic and Asian tribes displaced the Roman and Celtic tribes of Western Europe.
This is only challenging to understand if you falsely believe civilization and technological advancement to be socially preferable. Historically that is not the case. Civilization tends to emerge not of preference but out of unwanted necessity to regulate resource production/distribution.