Well, there's something
Nevertheless, you know, the reason is obvious - the setting is just too dirty. The NTR genre thrives on the corruption of something pure, and preferably in a hidden way. Contrasts are essential here.
That’s why, for example, a shining magical fantasy world with its vivid, fairy-tale tropes works exceptionally well for NTR. And it’s more effective than sci-fi or modern settings because fantasy inherently emphasizes the importance of individuality and the very idea of personal supremacy.
On the other hand, an apocalypse - especially the zombie variant - immediately plunges into filth, both metaphorically and literally. Let’s be honest, NTR in such a context is just a minor spice against the backdrop of mass killings, cannibalism, and the like. By the logic of contrasts, sweet romantic stories would actually fit much better in such a setting.
It’s no surprise, then, that a genre like wincest-harem have fared better in this kind of grimy backdrop, with
DeadMoon Survival and
Zombie's Retreat as the most prominent examples.