It may be due to the fact that there may have not been anywhere to expand to.
What I mean is, they may have no where for the technology to grow, they may have reached their technological peak. For example, the Hyperdrive might have been optimized so much that only small tweaking and quickening of the drive may be accomplished, no great advancement in Hyperspace technology existed.
Or it could be the fact that there was no need for it; they had all the technology they ever needed, and then some. They had the ability to heal the most grave wounds, via Bacta, Kolto, etc. They had superior droids, and they had access to the Entire Galaxy's energy. There may have not been any thought of creating more advancements.