TL is a hard concept to nail down. For me it is like '⌧ography' ... I just know it when I see it.
I travel fairly regularly in the Caribbean and Central America on cruises (It get's me away from phone contact with work). What is the TL of Mexico? I can start out in a tourist port and buy fine watches and electronics by the armload (finances allowing) ... then I can travel less than a km and find operating vehicles older than most of what is available in my local scrap yard ... then I can travel an hour by van to visit some ruins and pass a man carrying a bunch of bananas up the side of a mountain to a hut that looks straight out of the stone age (with a 50% chance that the roof will be made of rusty corrugated metal.) That's a real world place with people living from TL 1 to TL 7+ within an hour's drive. If I visit the city, I would say that Mexico is TL 7/8 (like Florida, where I live). If I visit the mountains, I would say Mexico is TL 1 to TL 5.
In Traveller [In My Traveller Universe], I use TL as a tool to describe a place. What follows may or may not apply to YTU and probably does not apply to the OTU, but it works for me.
TL is first and foremost a measure of average wealth. More money allows you to buy more and better stuff. Rural India may have access to imported Mercedes Benz S Class cars, but few of the people can afford any car at all, so the 'character' of the area is probably closer to TL 4-5 in transportation (both vehicles and animals). An IMTU world of TL 5 would probably be an area where one would expect to find both draft animals and internal combustion vehicles sharing the roads. Some places would have electricity and telephones, but not everyone would have it. You could import a TL 8 'cell phone' with advanced data capabilities and WiFi and Bluetooth, but you will not find cell towers throughout the world ... so unless you are within sight of the starport, you get no bars on your phone. When your mercenary plugs his laser backpack into an outlet in his hotel to recharge, he blows the fuse and the floor he is on goes black until they can replace the locally made glass fuse or reset the imported circuit breaker. 220V 3Phase electric service is not available in town and those buildings that do have power make due with 110V 1 Phase power ... the mercenary needs to buy a charging station to trickle in the 110V local power, store it in batteries and then provide the 220V power to charge the laser backpack. The charging station might need to draw power for 4 hours to recharge a backpack in 1 hour. This is just one example of how the TL of the world will impact your experience while visiting the world.
Viewing TL as a measure of general wealth of a world, changes the question in the opening post from "Why isn't everyone TL 14-15?" to "Why isn't everyone rich?" As population increases, it becomes much more expensive to raise the standard of living of every person ... and frankly, who has the incentive to try? Who has an incentive to bring two cars, cell phones, public schools, gated communities, Walmart, large screen TV, etc. to every poor and remote village on the Earth?
I like the thought that any specific place on a world could be up to 2 TLs higher or lower than the Average TL of the world. My hypothetical TL 5 world mentioned earlier would have rural villages and poor areas that felt closer to TL 3, and the University near the Hospital and Starport might feel closer to TL 7.
THE HIGH COST OF IMPORTS: A lesson from a friend in Jamaica ... He can buy any video projector that I can, but at 5 times the cost. The government does not want all of its hard currency pouring into other nations to purchase imported goods and drive the local economy into a recession, so "Luxury Goods" (defined as whatever the government says is a luxury good) have a 400% tax added to the price. So not only does he earn less money for the same job than I do, but he pays 5X as much for imported high tech goods. So for a TL 9 world, an air/raft might cost 4 years salary for the average worker and for a TL 5 world, importing that same TL 9 air/raft might cost 40 years salary for an average worker. In more practical terms, someone earning 4000 credits per month on a TL 9 world might just afford to 'drive' an air/raft, while only those earning over 40,000 credits per month on a TL 5 world could afford to 'drive' an imported air/raft. That does not mean that there will be no air/rafts on a TL 5 world, but it does mean that the streets will not be clogged with air/rafts on a TL 5 world ... which is why it is TL 5 and not TL 9.
Even if the world does not restrict imports with tariffs, exporting Resources is a hard way to pay the bills. Let's sell Special Alloys at the market price of Cr 200,000 per ton (or dTon, let's not split that hair). With TL 9+ laser mining and fusion powered processors, it probably costs about half that price to extract and refine the ore into marketable alloys. So at TL 9, it will cost you Cr 100,000 to produce 1 ton (or dTon) of Special Alloy that will allow you to purchase Cr 200,000 worth of imported goods. At TL 5, extraction and refining is more expensive and time consuming. Looking at fuel costs, TL 5 hydrocarbon fuel is 7 times as expensive by volume and over 570 times as expensive per MW as TL 9 Fusion. So that same 1 ton (or dTon) of Special Alloy that will allow you to import Cr 200,000 worth of Advanced Technology will cost you Cr 100,000 to produce at TL 9 and Cr 5,700,000 to produce at TL 5. Each 1 credit of imported technology will cost over 28 credits in local currency. That 100 credit Super-Widget that you see in an Imperial Catalog and really want to buy, will cost you Cr 2,850 worth of labor to purchase ... and if the government wants to engage in protectionism to avoid sucking all of the cash out of the economy, then the 100 credit Super-Widget will cost you Cr 2,850 plus an 11,400 credit tariff ...
Price in catalog = 100 credits.
Cost to you = 14,250 credits.
Don't get too bogged down with the specific numbers. They were based on one set of Traveller data and assumptions. Starting from another set of data and assumptions will yield different specific values. What is important ...
- TL is what material goods are dominant, which is a measure of wealth.
- Raw Materials can be exported to pay for high tech imports, but are more expensive to produce at lower TLs.
- Governments may not want all of their wealth flowing to another world and may use tariffs to reduce imports.
- Imported goods will always cost a lot more than the same goods at their point of manufacture.
- You cannot just raise one area much above the local TL before the need for infrastructure will require TL advancement in other areas (TL 9 lasers will not plug into TL 5 power grids without additional equipment).
Agree or disagree, but that is my contribution to the topic.
very hard to get a C class PUBLIC starport to Rethe. The members of a Shipping Cartel use their PRIVATE spaceports to move the goods needed by Rethe and they fight against improving the overcrowded public starport as a way to interfere with prospective competition. 
... I have about 16 billions peoples to put to work and I favor labor intensive processes...that is why I am a charismatic dictator
