Why is it so hard to find well-written zombie fiction?

Zombie fiction isn't even close to the only thing I write, but I find myself constantly on the lookout for some to read. In the last several books I've read (whether free or cheap) I keep running across the same pitfalls. I won't name names, because I don't feel it necessary to in order to get my point across, and I have no desire to trash another writer's work personally. I'll have my own detractors, of course, but there's no way in Hell that they'll be able to accuse me of the following sins.

I'll experiment with KDP Select, so you don't have to.

If you've paid attention at all, you know I'm a self-published author.

I've done it through three different publishing services. I originally published through Smashwords, because I liked the multiple distributions to outlets like Barnes & Noble, iBooks, Kobo, etc. It seemed an easy way to keep most everything in one location. I've since learned that I prefer to publish on Barnes & Noble directly through their own PubIt! service, because making price changes, for example, can take as long as a couple of weeks through Smashwords. And reporting takes forever. I honestly have no idea how well my first two books are doing on any of those minor (in comparison) outlets. In a nutshell, the PubIt! service makes it much easier to manage my books on B&N.