I think this is certainly part of it. For the most part, what we know about the bronze age is what was written down by the romans and greeks, with the rest of europe being essentially boiled down to barbarians and a small amount more known about the middle east. The fact that the groups we know the most about ran on slaves during this time period doesn't help from a modern perspective as well. It's one thing to have monarchies and nobles but they can still be portrayed in a positive manner from our point of view, but you can't really brush off the fact that the majority of people are slaves. Those well educated might have lived relatively decent lives as far as slaves went, but the fact was the vast majority lived short and brutal lives toiling in mines, mills, and farms until they died an early death. Hell, look at the Spartans everyone thinks are so cool, their entire society was basically an elite noble warrior caste and then dozens of slaves for every free man. They were known as zealous slavers even by Greek standards at the time.