Farscape wasn't a failure because it just recently had a mini-series come out. It's had its obstacles, but wasn't a failure.
Firefly wasn't a failure because, last I heard, it does have a movie coming out at some point. So, while it perhaps still has some obstacles, it doesn't look to be a failure.
As for Angel, well, I'm sure it has some kind of product tie-in's with Buffy that, while not television, are seeing fit that it continues in some form.
Not to mention that just because something is good, it still can be a failure. As has been pointed out a number of times throughout this thread, sometimes failure has absolutely nothing to do with the quality of the books themselves.
Also, both Midnight and Iron Kingdoms do have a main campaign setting book, although the one for Iron Kingdoms was rather late to hit the shelves.
The sad thing is, at this point, I think it might be too little, too late for a true, essentially self-contained Scarred Lands book to grab anyones interest. The main core of buyers would already have most everything in it, whereas those who didn't buy the setting to begin with have probably since already been turned off to the setting. Right about now, I'd say there's about zero chance of anything being published for the setting except perhaps in the most miniscule of areas. Maybe something akin to the way Wizards of the Coast has been treating Planescape, with an occasional blurb about it here and there in a planar book, but that's about it. Fan support currently seems the only avenue to do it with.