>>25625421
Historic and geographic proximity to main trade hubs and international trade lanes - the further, the poorer.
And then the effect is simply accumulates with time.
Like, if you earn $1K and your neighbour earns $900 in 1000 years you will be rich while your neighbour will be destitute as you both would have to pay for about the cost for goods and services if you both wants to keep the same level of technological and cultural development, but you would have more money left after buying the necessary. And then you can invest the money to get richer. While your neighbour would have to borrow money to keep up with you and so only his debts would pile up.
You can call it luck, in a way.
Though, its more complicated. For example, German tribes suffered a lot of genocide cases due to be more exposed to interactions with other tribes, while Slavs hid themselves deep into useless forest nobody wanted and nobody visited. So they better preserved themselves and overtake other tribes that lived in better places around the Slavs. But it cost Slavs opportunity to get any good geographical location to themselves when it was still possible.
So, it's luck but also a choice. And Slavs made a safer bet and won much less in the result.