DDM minis sell at least in part because (like all other minis games) there is a game that is directly connected to its minis sales- you can't play DDM without DDMs (not entirely true- I did some Confrontation conversions, but...) AND its directly connected to the biggest RPG in the history of the hobby. It has 2 groups of hobbyists who would want the minis.
Chainmail, DDM's immediate predecessor, suffered somewhat from the fact that it had many minis that were not (at the time) connected to D&D. I know players who bought lots of Drow, Humans, Orcs, etc., but wouldn't touch a Pulverizer, Equiceph, or Abyssal Maw. Release some of those today and they'd sell just fine.
Other minis games that have a mixed bag of units unique to their game and those that crossover into other games probably also see similar differences in sales- those that crossover probably have sales disproportionate to their actual use in playing the minis game.
Do you think the manufacturer doesn't have storage and management issues? Unless they only print what is ordered they have to store the balance between what has been ordered and what has been produced.
Sure they do, but there are numerous ways a manufacturer can control their inventories that don't work as well for retailers.
For instance, if Reaper used JIT (just in time) manufacture processes, they'd do just fine. They have the minis molds, so it could work for them. They send out an initial shipment of minis A-D, only manufacturing enough for that initial shipment (possibly a little extra), but keeping the raw materials on hand for manufacturing more (or not, if they use JIT materials ordering as well). If minis A, B, and D sell, but C doesn't, they simply don't manufacture C anymore. The material that would have been used to manufacture C can be shifted to manufacture A, B, or D, or even for the initial run of E-H (or if using JIT, it simply doesn't get ordered).
The material- the metals used to make the minis- don't care what mini they get used in. The main waste for the manufacturer are in the expenses that went into making the unsuccessful mold- but if that still makes breakeven, they're still OK.