Angelika Maier wrote: I wonder why this is not used more often.
As a guess, probably because it's not a significant cost savings. The two most expensive parts of normal concrete is the Portland cement and the labor. The cost of the aggregate (sand and gravel) is pretty minor compared to the rest.
Soil cement uses about the same amount of Portland cement and requires as much labor if not more, so you end up with a less durable product for only slightly less money. As a guess, if you contracted out a long driveway, you'd save maybe 5-7% going soil cement instead of concrete.
On the other hand, I've made soil cement for small projects and it looks just like dirt. So if you want a hard paved surface that blends in with the surroundings, it's hard to beat.
Note: If you're doing all the work yourself, and don't consider the cost of your labor, then the savings for soil cement work out to around 25%.