I suspect, FS, is that it's two fold:
1) It's just not something that folks think about. Tech and science and all advancing is A) a peripheral thing, something that happens in the background, and thus, is glossed over, adn B) easily forgotten about. When you're spending time working on "What is next week's adventure?" you don't worry about "What's going on in the rest of the world that doesn't relate to my plot(s) whatsoever?"
2) Keeping the status quo, and keeping the medieval feel. Folks don't want to change the setting from medieval to Renaissance/Industrial Age. They don't want gunpowder. They don't want to get away from knights on horseback in platemail. They want to keep it where it is so they can play what they want. Unless the PCs cause the changes in the world, then they don't want the world to change, period.
I've seen a few settings that advance things ahead. But that, I suspect, are the two reasons why campaign settings don't advance.
1) It's just not something that folks think about. Tech and science and all advancing is A) a peripheral thing, something that happens in the background, and thus, is glossed over, adn B) easily forgotten about. When you're spending time working on "What is next week's adventure?" you don't worry about "What's going on in the rest of the world that doesn't relate to my plot(s) whatsoever?"
2) Keeping the status quo, and keeping the medieval feel. Folks don't want to change the setting from medieval to Renaissance/Industrial Age. They don't want gunpowder. They don't want to get away from knights on horseback in platemail. They want to keep it where it is so they can play what they want. Unless the PCs cause the changes in the world, then they don't want the world to change, period.
I've seen a few settings that advance things ahead. But that, I suspect, are the two reasons why campaign settings don't advance.