For some reason, trade doesn't make much of a difference in the tech available on worlds above a certain population size. It accounts for only about 1/3 of 1 percent of the Marches GDP. At Porozlo, right next door to Rhylanor, it's a bit under 1%. It only starts being a significant fraction of GDP when you get down to populations in the tens of millions and less, and then only where they're well-positioned for trade. As near as I can tell, for the vast majority of Marches citizens, any imported tech amongst that trade is only making it to the "one-percenters" - the rich - and to a few lucky small-pop worlds. You probably can't get it if you don't live near the starport or have a lot of wealth and influence.
As you point out, there's every reason to believe tech trade should be a lot higher, but Rhylanor's exports only account for a bit more than half of one percent of its GDP, and Mora, Trin, and Rhylanor are similarly low. By comparison - although comparing real world values to a fictional far future setting is tricky - US exports run to about 13-14% of GDP. One would think worlds capable of advanced robotics would be capable of exporting a lot more stuff.
Canon notwithstanding, the trade statistics suggest there are some pretty stiff trade barriers being erected by the Imperial worlds.
