Well, maybe since the Republic, Sith, AND Jedi fell from their superpower positions the galaxy was plunged into our equivalent of a "Dark Age", where technological progress was stunted. If our own history proves anything it's that in the absence of a superpower, technological and social progress slows down, halts, or even degrades.
Another possible explanation is this: ever read Nineteen Eighty-Four? It seems that the Republic is in a constant state of war (Exar Kun, Mandalorian, Sith, and lord knows what in the next 4,000 years), so when expending all resources to the totally destructive and pointless state of war, actual technological progress doesn't really exist.
In any event, it IS just a game, and this arguement seems rather ridiculous, though debating such matters can be fun. The most likely explanation is to keep the familiarity of the Star Wars world, but in that case, I don't see why it was necessary to use such a ridiculous time difference as 4,000 years.