If Pilea caderei were a person, it would be a teenage boy: it wants to be everywhere at once, is more often in an awkward phase than not, and needs a grower who can set some boundaries once in a while, lest it throw wild drunken parties all night long.
It has some pretty specific ideas about exactly what it wants, too, with respect to lighting (bright indirect or filtered sun) and humidity (the more the better).1 So it's not that hard to grow, but it's hard to place: once you find a spot it likes, it'll grow, and grow, and eventually take the car on joyrides without your permission. (It's also difficult to place because you really need to be able to see it from above: the foliage is only patterned on top.)
Most of the difficulty is from the light and humidity needs, but, as I've hinted above, regular grooming is also more important than it is for most plants. Plants tend to get leggy pretty quickly, and sooner or later you reach a point where no amount of pruning is going to be enough. Fortunately, it's very easy to start new plants: I've started several at work, in the greenhouse, and had a success rate of around 90%; my track record at home is, if anything, better. And this is without doing anything special to the plants at all: cut a piece off, stick it in wet dirt, and wait, and nine times out of ten it'll be fine.
They're not enormously popular plants: I see them for sale occasionally in Lowe's and Wal-Mart and the like, but it's pretty rare. I suspect the deal is that although they're easy to produce in the first place, they have a very limited effective shelf life, due to their tendency to get leggy in a big hurry. In the home, this is totally controllable, and only takes the occasional pinching back or restarting, but pinching back takes time which could probably be more profitably directed elsewhere, and also the plant actually looks worse, not better, for a little while after a good pruning, so it would be hard for your average box store manager to see the point.2 Also, from a retail perspective, a plant is a plant, and if there's one type with a limited shelf life that needs a little extra work and one with a longer shelf life that can just sit there, you're going to sell the longer one. Period. Which is too bad, I think.
It may also be the case that Pilea cadierei is just not a "cool" plant right now. Fashions come and go in the plant world like anywhere else, and sometimes a plant can be hard to find for no other reason than that a lot of people are bored with it, particularly if it's difficult. If a plant is hot, though, it doesn't matter how difficult it is, they'll still find a way to sell them: majesty palms (Ravenea rivularis) aren't, I think, properly indoor plants at all, but boy, Lowe's always has some in anyway, and they must sell.3
I don't know for sure that mine are going to make it through the long, dry winter; I've got them in the bathroom (along with my Dizygotheca elegantissimas), and we'll see how it goes. If they don't do well once it gets dry and cold, then I suppose we have part of our explanation for why you don't see more of them. So far, I haven't had any noteworthy problems outside of a couple failed cuttings.
As far as work goes, we've sold a few, though they seem to move the best when they're very small and cheap: all the 3-inch pots I made up sold, while most of the 4-inch ones are still there. This may or may not mean anything, since I can't remember how many I made of each.
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Photo credits: me
1 (The plant is native to Vietnam rain forest, so you know there are going to be some arguments over humidity.)
2 (In my experience, it's difficult to make box store managers see any points, but perhaps now I've crossed the line to just being snobby.)
3 Another plant I suspect of being "uncool" at the moment is grape ivy, Cissus rhombifolia, which I see only very occasionally in stores. Customers asked me for it a lot, though, this summer, so maybe it's making a comeback. We got a lot of them in in the big mid-October tropical shipment, and a few have sold already, so perhaps. Stuff like wandering Jew (Tradescantia zebrina) is kind of doomed to perpetual uncoolness in the trade, because it's too easily grown and passed around from cuttings, and also it takes almost no time at all before leaves and stems start dying back and then the whole thing just looks messy: nobody really has the time to keep taking cuttings and planting them in the pot to fill it out again, though we try sometimes.