Is it because both groups are using material reality as the baseline? Rather than POMO 'reality is whatever you want it to be' nonsense. (I admit, as a scientist, I don't really understand what postmodernism is all about, but that's how it appears to me).
Yes, in the west I think that's a big factor. Religious conservatives aren't materialists, but they do think the physical world is real and substantial and also that it is rational. That's why the scientific method comes out of that perspective, or the medievals came up with the idea that gee, maybe we could apply mathematics to the study of the physical world.
The other overlap that's important is what you might call class analysis over individualism though they might not use that language. Liberal progressives tend to understand the need for legal and social controls in economics, but not really in the social sphere. So, they want something like pornography to be a question for individual freedoms.
Religious conservatives tend to see questions like that as impacting behaviours at a group level, and also impacting what they would call the formation of people's souls. Virtue is in part about good habits, both physical and mental, and people don't have those unless they are brought up to them - they are much harder for adults to acquire. They are also likely to see right thinking and habit in one area as impacting on people's behaviours in other areas. This isn't all that dissimilar to many rad fem ideas.