An interesting example is The Good Place, which is a show literally about heaven and yet is deliberately secular and non-religious.
A major factor in that is how TV writers are an overwhelming secular group of people.
This is basically it.
Honestly, it likely comes down to it being more difficult for an irreligious person to relate or understand a character that's going through complicated religious struggles, but everyone can more or less follow when shows grapple with more universal or secular morality conflicts.
Sure, but the "universality" point can be made (with varying degrees of convincingness) about any writing focusing on anyone who isn't a heterosexual, able-bodied, agnostic or very vaguely Christian white man, and I think what Averroes is asking is why there's a push to allow more "specificity" in storytelling along all of those axes but one.
I do think there is less nonwhite or LGBT specifity than one might expect today, even. But I think a lot of the uncomfortableness comes from the idea that religious belief is in itself more absolutist/exclusivist than say gender, which would make its depiction more problematic. Portraying someone's religious belief positively is open to the interpretation that the show is making a statement that
this is the right way to act or believe for everyone, which feels more controversial to showrunners than say about a struggle with say a character's personal sexuality ("gay agenda propaganda" conspiracies aside).
