I've run a couple of handfuls of campaigns in my time. Most of the them were short runs. Only once did I have a long run. But in all of them there was never a true 'gaming' history to my worlds. I'd make up something new each time I ran a game. So that sense of history isn't there.
Rob Conley over at Bat in the Attic has been running his Majestic Wilderlands campaign over thirty years now. When you start playing in City-State you know you are walking into a place neck deep in history and he knows it all. Rob doesn't hit you with exposition of info dumps, he lets you experience it. When you go into the tavern it bears the scars from the previous adventurers that have come through. When you are dealing with a guild master you know their is a organization behind him based on guild laws pasted on for ages. Walking across the landscape you can feel the ghosts of the soldiers who fought on this land. This is not hyperbole. This is what I experience when I game with Rob.
Many campaigns, including my own, lack this depth, lack the character only age can bring. Only playing it year after year with different people at different times can it be developed. I know my Monday night campaign will be coming to a close soon and Rob will start a new Majestic Wilderlands game. I'm looking forward to skulking through the streets again. I'm probably one of the people who has played through it the most and each time I play I still find something new. I still find things that surprise me. How many campaigns can say that after 30 years?