Let me start by pointing out that Ral Partha was eventually bought by FASA, and when they closed down the ranges were made by Ironwind Metals in the US and Ral Partha Europe in... Well, in Europe ;-) They still sell a lot of the old stuff/ranges.
Perhaps the market will only support a few producers of non-randomized minis. Perhaps there have been so many mini companies pumping out product for so long that non-random minis are not as desirable when they are not tied to some other product.
By this I mean that nearly every mini company puts out "the basics": dwarves (esp. w/axes or hammers), elves (esp. with bows or longswords), humans, skeletons, orcs of some kind and so forth... Well, how many of those do you really need?
I think you have a couple of factors that play a roll:
a.) Competence, a lot of the smaller miniature companies have closed due to incompetence of management, folks that are either hobbyists at hart, business folk that have absolutely no idea how the hobby works, or even a few cases of robber baron mentality.
b.) Lack of innovation, or more precisely lack of imagination. You often see that when a range is almost complete, the company spirals down very fast because they don't really know what to do next. The reason why GamesWorkshop/Citadel are/is still around with Warhammer Fantasy Battle for almost 30 years is because the revamp/redesign the entire range at regular intervals and having a wide range of miniatures. The reason why Rackham failed is because when the metal ranges were almost complete they didn't start revamping them one by one, but pulled the plug and did the same thing over again just in a plastic prepainted medium. Old fans got pissed and had no real desire to buy the 'new' miniatures, new fans had to step into a 'brand new' game whose ranges were very limited, not to mention a hostile community.
c.) Company doesn't scale well. I've seen excellent small companies grow rather fast only to stumble and fall flat on their faces when the company didn't scale properly. Companies can either not fill demand and folks get pissed and go to the competition and eventually you'll loose almost all your customers due to a bad reputation in that department (recently happened to Privateer Press). Or the companies make the (relative) huge investments to keep the company growing, but the growth doesn't scale with the expenses that were made (and still are being made), so the company goes broke. You could file this under bad management, but imho it's not about being stupid, but rather that the business model will not work at the time.
I think that the miniatures market is actually a bucket that does grow and has a small hole in the bottom, but the influx is rather larger then the growth and the drain combined. GW especially is feeling that pain on the secondary market. When they put out a mini into the world, it isn't used up, it doesn't spoil and doesn't really become obsolete, the only real losses are purposeful destruction of loss (in the attic). Even most damaged miniatures can be repaired. So when you have a complete army you don't really need a new one of the same kind, the only solution is that you need a bigger army and/or new units. This is exactly what GW has been doing the last 1-2 years or so. But there will come a time when GW is going to loose the battle, miniature demand is saturated, larger armies are not feasible due to physical space on the table, and the business can't support a deeper product range (GW is already combating the last, a single product that can be build into three different types of monsters/units).
I think Paizo/Wizkids will also eventually hit a saturation point. If the announcement is any indication we'll have two sets per year, resulting in 100-120 minis a year. Releasing sets to coincide with Adventure Paths is really smart and should make it possible to support the mini range far longer then normally (if quality/price/range is right). On the other hand, to date we've seen a 300+ monster bestiary every year now, I'm curious if they can keep that up for a decade.
One issue that a lot of folks seem to forget is customers storing the stuff and actually finding what they need on short notice. I'm storing my CMG stuf in multiple rolling crates, registering all those minis almost feels like a job. The plastic stuf that I need to assemble and paint myself I keep in many, many boxes. And the metal stuff needs something even sturdier, certain companies take a lot om money from me on the storage front...