There's actually lots of good reasons why all starships would be required to do their business at the highport and why downports are really a bad idea. I'm not denying that downports wouldn't exist; probably on poorer worlds (regardless of cause -- "less populated" would count) but I think as soon as a world could afford it, they'd build a highport and confine trader traffic to it.
1. No need to streamline ships. Streamlining a ship involves a lot of trade-offs involving the dimensions of the cargo area and how easy it is to load and unload stuff from it, all of which ultimately have an effect on what the ship can carry. If all traffic is conducted at the highport, there's no need for streamlining. Ships could be designed as basically big RO-RO cargo compartments with the other stuff sort of tacked on, or more likely, you're going to see large, efficient "corncob" style container ships where the cargo is attached to the outside of the hull in standardized containers which are then unloaded using smaller ships vessels (really the futuristic equivalent of cargo cranes and forklifts more than starships). With ships like that, it might be efficient to have a queue of cargo ships in a long orbit to a colossal "unloading area" which is basically a huge free-orbit "tube" or "ring" type structure that ships pull into, and lots of spindly robot arms simply pull the containers off in a few minutes. The arms then sort the cargo into lots which are then grabbed by other EVA bugs for customs inspection, set aside to wait for the cargo ship that will take them further towards their destination, or whatever.
2. You can make people pay for fuel. I won't go into the mathematics, since I think there's a few posts on Mongoose's boards that discuss this already, but assuming a layout like our solar system, getting from Earth to Jupiter for "free" fuel is going to be faster using an in-system jump than it is to use maneuver drive. Time is money. That week you spend you Jumping out to the Jupiter equivalent could be spent Jumping out to another port of call and trading and (presumably) making money. People will pay the "outrageous" costs of starport fuel because that expense is less than the time lost Jumping out to a gas giant for "free" fuel (which then has to refined, etc.).
3. You can also make people pay for life support.
4. Quarantine. If your Traveller Universe has any vague concerns about diseases and so forth, you want to leave all that up there in the orbital facility where it can be more easily controlled.
5. Environmental Concerns. It's much easier to monitor and control starships so they don't dump their garbage, suck up your seas and lakes for fuel, bring strange vacuum-resistant organisms or just toxins stuck to the hull which then flake off and pollute your world.
6. It's really cheap to get stuff down the gravity well, likely even cheaper than bringing stuff up, even with gravitic tech. You can drop stuff quickly down the well since gravity does all the work for you. The only thing you might need gliders, parachute, space elevators, or (most likely) gravitics for is to slow the package down so it doesn't burn up on re-entry or make a big crater when it hits the ground.
7. "People don't like to live in big metal tubes." This works to your advantage. Those seedy, shifty trader types who become pirates or mercenaries at the drop of a hat will do their business, unwind at your licensed facilities in the highport, but won't stick around any longer than that because highports are all kind of the same. They can practice their murder and perversion there on the station. Also, if a universe has cheap, safe grav-tech and it is reasonably efficient, if it takes a hour to get into orbit or thereabouts, it's very likely that starport employees would simply commute to work from their homes dirtside. I know people with 1 hour commutes (or "close enough" given traffic) IRL. I know people with 1+ hours commutes. You probably do as well. You may be one of them.
