As far as I know, the ratio by which a layer is inhabited depends on two factors:
- how unpleasant it is (many are too bad even for the fiends - crushing gravity, temporal and magical anomalies, you name it)
- how hard it is to access (a large part of the challenge in Planescape was getting to your destination. Thus, getting to the first layer can be done by rather simple spells but beyond that, you need to either use much stronger magic or the right portal. Obviously, entering portals at random is risky stuff, especially since many of them are one-way.
Also, it is very hard to build a cosy resting place in the Abyss. As a place of evil and chaos, where the very soil is literally made of digested souls of psychos, it isn't very conductive to rest, healing magic and the like.
And if you actually manage to change a whole layer to your liking... it will no longer be part of the Abyss. And surprisingly, that's bad news too.
There was one high level adventure for Planescape where a "lawful good" but completely insane deity would condemn its followers for minor offences, using one layer of the abyss as its personal hell. It caused a planar disaster when the layer, placed in the Abyss but filled with mostly good guys, started to shift to its "proper" place on the great circle, straining the dimensional barriers of all the planes along the way, even as far as Sigil itself.