Well, take this scene, for example...
There is supposed to be a complex sky here. However, the number of unique tiles in this area required me to remove it to fit in the normal background details. Here, it wasn't such a big deal to remove it, as there is still plenty of detail. But these are two different areas... the volcano section and the part with the 45 degree slopes are one area, this tree section is the next area. There should be still space in the tree section to add in the sky and clouds, but even as such, it's going to require a lot of additional unique tiles to fill in said level of detail. We do what we can to make things as detailed as is feasible, but when you're dealing with a system with only one background plane and have graphics clearly intended for more than one, you have to do a lot of additional work to make things go together properly. A single variation change on something as seemingly simple as a tree branch will cost you numerous additional tiles, and that's for just one variation... the sky has numerous potential variations in it, meaning that the tile count will multiply.
