No offense, but I really don't think you understand the miniature wargaming market.
The randomized plastic miniatures were part of a product line that supported a skirmish level wargame. Any answer for "why the D&D miniature line failed" is futile unless it begins with asking why the D&D miniature wargame was successful for a while, and then died out. That was the primary market. That's the thing that died. The miniatures themselves are still out there.
Personally, I suspect it was just time. Miniature wargames tend to have a life expectancy much like roleplaying games.
The randomized plastic miniatures were part of a product line that supported a skirmish level wargame. Any answer for "why the D&D miniature line failed" is futile unless it begins with asking why the D&D miniature wargame was successful for a while, and then died out. That was the primary market. That's the thing that died. The miniatures themselves are still out there.
Personally, I suspect it was just time. Miniature wargames tend to have a life expectancy much like roleplaying games.