An example of what I'm referring to is this image of Mithradates VI and Hypsicratea. He's a lot less "Greco-Persian Monarch" and a lot more "Henry Tudor" than I would have expected. Was this because of a lack of reference for what those people and locations would have looked like? Or was this a more explicit decision to try and relate them to the current events at the time? Was there just not a general understanding of how different the environment, technology and culture would have been for these people and events?
Take a look at this section from the popular questions section of the wiki talking about how people in the past viewed the future. The broad answer to that particular question is that, before the Industrial Revolution, life changed very slowly or not at all. It was difficult or impossible to observe technological and cultural changes within a single lifetime.
How does this answer your question? Well, let's extrapolate backward: How would someone in 12th Century Saxony have imagined Julius Caesar? How would someone in 8th Century Asturias have imagined the Biblical siege at Jericho? The answer: "why, things would be just the same then as they are now! After all, why would anything be different?"
So: it works backward as well as forward. If it's impossible for you to conceive of a world where anything could change then you're unlikely to imagine that the past was "different" in some meaningful or even superficial way (clothing, weaponry, architecture, etc).
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