It had elements of capitalism, like private ownership and focus on trade via monetary means, but its economy was largely based on slavery and your position within the system was based almost entirely on where you were born and to whom. It's a pre-industrial system, capitalism isn't really the correct description for it.
I mean, obviously not? History is way more complicated than this, just because certain elements fit doesn't mean we look at it and go "yeah that's capitalism mate". Historians generally use capitalism as a description for economic systems from 19th century onwards - before then the correct answer is usually "it's complicated". I appreciate that can be frustrating if we just want to slap a simple recognizable label on things, but history doesn't always fit what we want it to be.
