economics. if a tech 10 agricultural world can make widgets at 10Cr ea, but a nearby tech 8 industrialized world can make those same widgets for 4Cr ea and ship them for 1Cr ea, then guess who sells the widgets and who buys the widgets?
Far fewer than the Industrialized world would like, because Cr7 of opportunity cost and jingoism cancels a huge amount of trade value.
For example: I could buy Idaho grown potatoes for $4 a bag... but I can buy fresher Alaska Grown potatoes for $5 a bag. Or spend $5 in gas to buy them for $4/bag at the farm.
Yes, some Idaho potatoes get sold in Alaska... but many people are shocked that Alaska potatoes outsell them amongst the poorest neighborhoods.
Likewise, if I want a custom computer, I can have it tomorrow by going down to the local guru. I'll pay about 25% more, but have it tomorrow, versus some point next month. And that's not accounting for the warranty issues of external purchase.
There are literally thousands of exemplars disproving the "the lower price always gets the sale"...
The big reasons:
- Opportunity cost - minimum 2 week delay in the OTU to order out
- Local Sourcing Preference - usually citing
- Local Job Creation
- Local self-sustenance capability
- Jingoism and Xenophobia
- Local cosmetic variants
- sufficiency of supply - if A needs more widgets than B can offer, A will make them locally to make up the difference
- Brand Loyalty
- Local Seasonality (usually an issue with produce, not dry goods, but seasonal demand for dry goods...)
- Transport availability - it's much easier to arrange on-world transports in most cases
- Simple hassles of dealing with remote suppliers
I'm not saying there won't be trade, but there are many reasons why people on A will not buy I's widgets, even tho I's are less expensive.
And, as we in the real world start crossing firmly into late TL8, possibly TL9, we also have the issue of rapid manufacture devices... we're starting to see a lot of ordered-out industrial stuff being replaced with local CNC Lathe/Router/Mill machines and big blocks of remote acquired metal. In some strange places, no less. Like a Machine Shop in Tok. And a major CNC machine in Seward (there's a repair yard there), and one slated to go in at Dutch Harbor. I'm told there's one in Kodiak, too.
The "Hassle factor" opportunity cost often outweighs the actual strict opportunity cost (which is how much do you lose by not getting it sooner).
There's also the issue of on-world shipping in Traveller... It's not just how much does it cost to get the part on-world, but you still have to distribute it on-world... If A's manufactury is decentralized, I's goods have to compete with local sourced by adding not only the Cr1 to get it across the black, but also the Cr1 to Cr2 to ship it from the starport to the local consumers' warehouses.
It takes a surprisingly steep differential to cause a shutdown, in the modern world at least, and the 3I is comprised of a large majority of people living on worlds that have local histories including near total loss of interstellar trade and the devastations that it can cause.
There should be a lot more trade paranoia than many want to give credit for. (Yes, another intentional pun.)