I tried to tackle that problem in both SoI and WotW background, why there is little traffic between systems in a galactic civilisation spanning several star systems:
Basically it boiled down to the following points - spoiler tagged the details:
1) Terraforming/Colonizing a planet is a delicate process and implanted ecosystems by their very nature discourage travel from one planet to another.
Note: Terraforming here extends beyond just makeing a planet habitable for human life but also to creating an ecosystem on the planet that works for human designs.
Through the magictechnology of genetic engineering (and to avoid any (more) major terraforming mishaps [see 'The Chronciles Of Solace's' Mars]) most colonies are self-stustaining when it comes to flora and fauna providing food, oxygen and waste reclaimation. Technical advancements have made importing animal life beyond the scale of insects unnecessary, everything you could need a beast of bruden for you can get with a motorized vehicle.
Leaving a colony is essentially dangerous for the destination colony and travelers coming and going are a constant hazard to the individual ecosystems transplanted on the colony worlds. Unwillingly migrating organisms designed to thrive outside your colony could endanger another colony and tigger an effect known as Contraction (again term borrowed from the 'The Chronciles Of Solace'). A Contraction event would trigger a chain reaction that would endanger all settled planets and so it is kept in the mind of entire GTVA populace that such an event could be tiggered by the slightest of oversights.
2) If a ressource cannot be naturally found at a colony site, the colony's society, in the long run, finds a way to substitue the ressource with something readily avalible or easily optainable.
self-explainatory
3) Economic policy is to restrict production to products that can be manufactured with local supplies in order to limit foreign control over policies in a system.
Most economies (note: plural) in GTVA space are not interdependent after a period of gaining access to the ressources avalible in system. Long term dependence also means that on the long term someone else has you under their power (see Oil Crisis in 1973). This is also a remnant from pre-GTVA-time when there was not a single but multiple competing power blocks.
4) In relation to 2 and 3 there is a small number of industrial sites in the GTVA space which have a demand excedding their supply in system.
self-explainatory
5) In-system jump drives come at different levels of capacity and different price tags. The laws governing subspace flight make subspace jumps economical only for small ships.
DISCLAIMER: The following is purely fiction and may not have solid foundation in contemporary science. Anyone to point out the faults and correct them is awared a metaphorical cookie.
Basically a subspace jump is easiest to accomplish when space is already bend by a mass rich object (e.g. a planet) in close proximity to the entrance point of the subspace transit. Moving away from a planet increases energy consumption for the transit. Mass of the ship transfering from point A to point B also factors in the energy consumption of the transit. Transit lengh which is the capacity to maintain the subspace transit is the final limiting factor as the longer the transit the more energy is consumed.
Hence jump drives have the least energy consumption on small ships makeing short jumps from near a mass-rich object. To jump larger ships, longer distances and/or from anywhere more energy is to be fed into the jump drive and so the ship needs a larger powerplant to achive this. Large powerplants, even fusion power sources, are expensive to make and maintain which raises the cost for their use.
6) Intersystem drives are really expensive.
DISCLAIMER: The following is purely fiction and may not have solid foundation in contemporary science. Anyone to point out the faults and correct them is awared a metaphorical cookie.
Jump Notes allow subspace entry at severly reduced energy cost due to the spacial properties at their site. However intersystem jump still take up a lot of power due to the lengh of the jump and therefor a ship with a intersystem drive must either power down power consuming systems for the transit or be able to supply enough power for the jump to be successful. Special issue drives, like in military fighters, are able to minimize the problem somewhat but are a huge investment in themselves.
7) In relation to 2-6 intersystem commerce is expensive, promises little returns and so has a slim profit margin.
self-explainatory
8) Ships qualifying for commercial intersystem flight have to be build with a life-time in mind that returns the investment. Building and maintaining ships with long lifetimes is expensive.
A ship with a long lifetime must be able to be refitted easily and often in order to ensure it remains competetive with other vessels with the same purpose. Refitting ships takes time and money additionally to the orignal investment for the construction. The original investment is increased by taking above factors into account.
9) Due to the high costs involved intersystem commerce is highly monoploized and competition non-existiant.
A company that succeeds can remain successful in the long run while company that stumbles and fails may already be finished with the first problems.
10) The monopolies on intersystem commerce are beneficial to the GTVA and thus are maintained by the government.
In accordance to 3 the GTVA has little economic leverage against its member system other than restrictions that could be imposed on interstellar and -system traffic. Limiting the list of the people involved here allows the GTVA for much more efficient application of force.
To summarize:
Interplanetary travel is discourage by an apocalytic terraforming catastrophy. Intersystem commerce is expensive, rare and as a result monoploized. The monopolies are beneficial to the central GTVA government as they allow the GTVA to apply its limited power against its member systems more effectively.
Note: I have not really accounted for piracy because I kinda don't find a logical scenario for space piracy...
EDIT: Well 5 & 6 are WotW-specifica since there I had to come up with a lot of ideas on how to make subspace travel logically work ... I might have strayed from :v:'s orignal concept quite a lot there
but the rest of the pattern should be transferable I guess...