Because that's the way things work! It takes several years of advances to push a bus.
AGP4x was introduced in the TNT2 days. We sat through the GeForce, GeForce2, the GeForce3 was the first one to show a measurable difference between 2x and 4x, and the GeForce4 Ti4200 was notable for showing a noticeable difference (as in, a few frames per second) between 2x and 4x. Then there's AGP8x, today's fastest AGP cards show pathetically small differences between 4x and 8x. 8x has been on the market for how long?
So far advancement has been at a predictable rate, my comment makes a very safe assumption that it will remain at a predictable rate.