The seeming stagnation is due mostly to authorial intent. Lucas's fantasy galaxy is like Tolkein's Middle Earth. It's a fun way to keep your universe basically unchanging. Sure the government changes hands, but everything pretty much stays the same. The technology changes only in matters of degree. Bigger fleets, bigger ships, more guns, bigger armies. The Death Star is like those little ball turrets on the LAAT, just scaled up by a huge margin, with the necessary power generation and defenses all there for you. A lightsaber is just a frozen blaster bolt or some such fantasy science.
Besides, the Star Wars stories only take place over a period of 35 years, tops. Where you get into problems is the Tales of the Jedi series which takes place some 4,000+ years before the movies. They decided to make everything the same as it was in the movies, only dressing people more like New Gods (DC comics series) and giving their tech the decorations of ancient Rome or Egypt (like Stargate). They have all the same stuff.. hyperdrive, lightsabers, blasters, etc. I can't really recall any technology in the movies that they didn't have in TOTJ (and KOTOR). They have planet destroying super weapons too. I don't care if they want to call it some "sith artifact" or what, the point is, it does the same thing. Either they lacked imagination, or they worried that people would not recognize it as Star Wars unless they put in all the same conventions, but they wanted to set it far enough in the past so that they wouldn't have to worry about stomping on the continuity of any of the established characters (of course you get some bungling with the history of the Sith and Republic, but oh well, retcon cannons at the ready, firing with both barrels!).