It's not necessarily the main reason, but it's a reason. Tons of stories, I'd even argue that it's most, don't even venture into someone's sexuality, race and neurotypicality. Readers generally don't care, as long as it isn't shoehorned in just for the sake of ticking boxes because you want to have a vibrant cast.

All these characteristics bring their own baggage with them, and thus conflict. If you're focusing on a story about a parent choosing to move on from losing their partner and having to remarry to make ends meet, sacrifice their own desire for love for the well-being and future prospects of their children, just as an example for conflict. Then giving the character autism, or an ethnicity that's being discriminated against in that society, or any other characteristics; then it might take away from, diminish or even undermine the story you want to tell.

In longer stories you can have different character arcs that allow for more nuanced characters and sub-plots. Weaving all of that together in a cohesive whole is a skill not many writers possess. Heck, if you've ever frequented fiction aggregate websites you'd know that the top user created lists are collections of stories that lack any and all romance.