... Ah, but those are four high-population TL 15 worlds, fully capable of producing all the spare parts needed to keep a lot of TL 15 starships flying ...
So the interstellar traffic of 272 Imperial worlds with some 256 billion people between them rests on Glisten, Rhylanor, Mora and Trin, the closest of them 10 parsecs from Regina and 18 from Jewell, while the industries of Fournice, Porozlo, Mire, Strouden, Lunion, Efate and other worlds can only look on and be jealous.
Problem 1: canon. Canon notes a number of shipyards at a number of worlds, and says shipyards exist at A and B ports. Imperial Encyclopedia, for example, notes the General yards at Regina, Efate and Pixie (Pixie??? What, is it robotic?). What are these yards doing if all ships are built to TL 15 out of 4 systems? But ... the rules stipulate a TL 15 ship is built at a TL 15 yard.
Problem 2: repairs. Your car needs spark plugs, you can get them almost anywhere, almost any time. Your car needs a damaged cylinder head replaced - that gets a bit more complicated. Routine maintenance, per rule, can be done at Class A or B ports: "Such maintenance ... requires two weeks at a class A or B starport." (Imperial Encyclopedia, pg 89) You still have the basic problem that major repairs will need major parts. Laser shot damaged your maneuver drive? Does the starport have a "cylinder head"? They might - free traders and scouts are pretty common - but if they don't, it's 5 months to request it and have it get to you.
Problem 3: Fournice, Porozlo, et al. There are 28 Imperial worlds of a billion or more population. 9 of them have the tech and starport to compete with the four TL 15s in the shipbuilding industry: Fournice (TL 12), Porozlo (11), Mire (11), Strouden (13), Lunion (13), Efate (13), Vilis (10), Jewell (12), and Palique (14). Of the Imperial hundred-million-pop worlds, an additional 4 meet the requirements: Ffudh (13), Edenelt (11), Adabicci (11), and Regina (12). Does anyone think the commercial interests of those worlds are going to let four worlds monopolize the shipbuilding industry? Let four worlds provide parts for 43 scattered Class-A Imperial starports? Let four worlds construct the sector's entire commercial fleet?
On the other hand, who pays twice as much for an inferior ship, when a better one can be built and shipped to you for less?
Option A: tech level does not apply to shipbuilding. It's more profitable for a world to import equipment for a TL 15 fusion reactor construction factory and a bonded-superdense-armor mill than to see that business go elsewhere, especially when most of the ship's other equipment can be manufactured locally. And, it serves the Imperium's strategic interests. Does anyone doubt the Duke of Regina wouldn't apply Imperial subsidies to help a Regina-based shipyard acquire the equipment to compete with the yards at distant Trin? Kindly ignore the rules.
Option B: tech level does not apply to ship repair, but only the TL 15 worlds build them. Same basic argument as above, the other worlds are importing equipment to manufacture parts - they just can't work high-tech armor, so can't build hulls (or complete power plants, if those are using bonded superdense to help achieve the smaller sizes). Kindly ignore canon.
Option C: all gods' Class-A children make ships, and at their local tech level. Tech level DOES apply to ship repair, and people ARE buying inferior ships at twice the cost because the starports can't import the equipment to do better - no one wants to find themselves stranded on Porozlo with a malfunctioning TL 15 power plant. Kindly ignore common sense.