I just found this fantastic interviews with some of the main creators of Planescape
www.expositionbreak.com
The whole blog post is worth checking out, but some interesting tidbits:

A Walk Through the Planes - An Interview with Monte Cook, Ray Vallese, and Colin McComb - A Walk Through the Planes - Exposition Break
A few weeks back, I was surprised to learn that a handful of Planescape’s original designers had stumbled into this site and taken a look at some of our coverage. Needless to say, I was ecstatic about this news, and used it as an excuse to reach out and see if they might be interested...

The whole blog post is worth checking out, but some interesting tidbits:
Monte: I think a lot of people were intimidated by it (in fact, I know they were because they told me so) because it was so different and so intricate. But different and intricate are my wheelhouse so it was perfect for me.
What most people don’t know is that much of Sigil was Zeb’s metaphorical analog for TSR itself. I mean, it was literally run by the Lady of Pain, whose very gaze could kill you (or your product). So behind the scenes, the setting had a very different meaning for everyone there.
Colin: As Monte says above, we all wanted more playtesting but… well, while we all had weekly game nights with cross sections of the company, the volume of material every team produced made testing it all simply impossible. The larger products tended to get more love because we had more time, but small adventures? We just had to go with our guts.
Sales had a big influence on later products. The big (expensive) boxed sets did not sell well. Dead Gods, originally, was meant to be a boxed set very similar to Hellbound, for example. And then, of course, the biggest one was that Faction War wasn’t at all meant to be the final product! There was supposed to be a product that followed on its heels that sort of put everything back together again (but in a different way). We were coming up with a few new factions, existing factions had merged, etc.
I’ll add that fans of those other worlds definitely didn’t want us there either. During the time when Greyhawk was about to be canceled—and possibly resurrected—I tossed a line into On Hallowed Ground about Oerth: “Chant is, Oerth is dying anyway.” It was intended to be an acknowledgement of their discussions on their message boards, but man, did they flip out on me online for daring to express this.
Monte: I felt very limited by the traditional D&D alignments and their corresponding planes. I was much more interested in the weird new places we could go and kind of bored with whatever was going on on Mount Olympus or some other more traditional realms. I had dreams of expanding the cosmology beyond the Great Wheel, which I liked for its elegance but didn’t like for its boundaries. I have memories of pushing for the introduction of an entirely new plane that’s just been discovered. (Ironically, the place where that actually happened was in Bruce Cordell’s Gates of Firestorm Peak where he added the Far Realm without any reference to Planescape or the Great Wheel at all, mostly because he was a brand new designer at the time and literally didn’t know any better. And the Far Realm became a big deal over the years—because of course it did, it was really cool and new.)
Colin: I’m probably too biased to answer this question, but I was really proud of our work, and I desperately hoped it would help keep the line alive. Given the disparity of audience sizes between the tabletop markets and the computer markets, I’m not really surprised. Torment’s sales were “disappointing” at launch, and wound up selling “only” 400-500,000 in its first year. I never got concrete sales numbers for the Planescape line, but I recall someone mentioning the first boxed set did well at around 75,000.
Colin: At Gen Con a few years back, Tony DiTerlizzi and I were talking about what we’d do if they offered, and agreed we’d want full creative control and some ownership. I don’t see that happening.
As Monte and Ray said, though, I’m not sure a setting revival is necessary. We had the extreme good fortune to be there with amazing people, developing a setting that has become legendary (can I say ‘legendary’? screw it, I’m gonna say it). Planescape laid the foundation for other projects (directly or not), helped people redefine what’s possible with fantasy, and opened eyes and imaginations beyond what we imagined was possible.
I’d love to return to the planes—working with the Planescape team was such a wonderful, formative, amazing experience. But I’d want to make sure we were making something truly creative on its own merits, rather than an appeal to old glories.