TL is a world's capability to produce goods at a particular tech level. I know you guys all know this, but bear with me. It's easier to conceptualize if you think of the worlds as different locations in a country.
In Southern California you have Lockheed, etc. producing let's say TL 8 aircraft.
In the San Francisco Bay Area, you have Silicon Valley, which used to produce computers and other electronics before manufacturing was sent overseas. Let's be charitable and say it produces TL 8 computers, electronics, and software.
In Western Washington you have Boeing producing TL 8 aircraft and Microsoft producing "software".
In Reno, NV you have Tesla producing TL 8 cars.
Elko, NV can't produce any aircraft, cars, or electronics, so it imports them from the regions (worlds) that do.
Elko, NV isn't TL 8, because it can't produce those products, but people live a TL 8 lifestyle because they can freely import those products.
Elko pays for its imports with revenue it gains from exporting its resources from mining.
If trade stopped for an extended period of time, Elko would be forced to rely on its own industrial and technological base, which initially would be TL 3 with the remnants of TL 8, and after 10 to 20 years of building infrastructure and physical economy it would probably be TL 4 or 5. Then it would begin the long R&D climb to higher tech levels.
Reasons why Elko would remain at TL 5:
Population or labor shortage. People work in the mines and on the farms, and there aren't many people who have the time, money, or inclination to do research for its own sake.
Resource scarcity. Commodities necessary for technological advancement, like fuels or rare earth minerals, exist only in limited quantities. Energy could be limited, and needed for production of export commodities.
Knowledge scarcity. Nobody has the knowledge required for technological development. Nobody knows the math required to research complex electronics, or physics, or nobody knows more than the fundamentals of metallurgy or chemistry. Nobody knows so nobody can teach.
Not Financially Practical. Nobody has the money to build higher tech production capacity, and it wouldn't be profitable anyway. Nobody needs TL 8 cars or smartphones, and they don't have the money to buy them. They can't be exported because high tech worlds are already flooding export markets with cheap mass produced products.
Nobody Cares. People are content with living or surviving how they are, and consider striving after unattainable goals to be pointless and wasteful. Or there are cultural obstacles, like spending money on entertainment instead of saving it, or having an obligation to support relatives, or there being a general preference for physical productive labor over intellectual or academic pursuits. There could be political problems, like the ruling elite wants to keep everything the way it is, and higher tech facilities would cause unemployment, discontent, or leave the population with enough leisure time to think about the problems with the political system.
And in a Traveller context:
Initial investment and infrastructure buildup is only for a single economic purpose. In Traveller, it seems reasonable that many worlds were settled for particular purposes, like resource extraction, and nothing else would be built except as necessary to support that primary purpose. Many worlds are probably the Charted Space equivalent of company towns or desolate impoverished miserable former company towns. They wouldn't even become ghost towns because people couldn't leave. The only ghost towns (worlds, colonies) would be those in which most people or everyone has died out. Failure of life support infrastructure, epidemics, civil strife, despair, gradual emigration until no one young enough to have families is left.