Outsourcing runaway.

It may be beneficial for the advanced civilization to outsource all manufacturing to some less civilized place like Earth for instance, where work is cheaper, work safety worse, ecology can be more dirty and these locals still have some brains, even if much inferior. Lower level of general civilization may not matter much, as the qualified engineers, first machines and ready designs would come, making the production possible. Later, however, education will start catching up, making outsourcing even more attractive, as cheaper locals will be able to replace larger and larger proportion of the foreign engineers.

Eventually all manufacturing may move into that cheap and dirty planet, closing the former factories first, and then also moving the research departments closer to production. Ultimately only the managers may remain in the former place, but also management can be outsourced and is more efficiently when not done through interstellar distances. Hence eventually we get one civilization making everything for another, just on the basis of some agreements signed generations ago, and few very rich representatives of the once-advanced civilization that still benefit a lot from all this. And the advanced race failed to notice that not just they passed over all technology they have to that they thought it is just a planet of slaves - they have lost this technology themselves. 

It is not difficult to predict that comes next.