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I feel like one of the problems with zombie figures is that you want to be able to army build them, yet with some exceptions like GI Joe's zombie Viper the zombies shouldn't look the same. Not like other army builders where you can buy the same figure over and over. So you have to release a bunch of different molds, but that eats up budget and there's probably a lot of releases that wouldn't sell as well as well. So gory stuff and figures of popular characters may sell, but the more mundane stuff probably doesn't, and in the end the figures just aren't that good anyway so its not like the boring designs can be saved by the engineering.
Here's a random example I googled, the zombie figures here are pretty boring compared to the more gory ones. I'm sure these were a threepack or something because I can't imagine them selling on their own.