A lot of people attribute this to a disposability of these people in the minds of writers, and in many cases that’s absolutely true. (Steven fucking Moffatt comes to mind. :-P) I do think that everybody is at least a little bit racist and sexist and queer-phobic because of the society we live in; nobody escapes it, we’ve just all got to do the best we can to overcome it at all times. While there are definitely prejudiced people who indulge their biases blatantly and without effort to correct, I also believe there are many people for whom, due to the fact that it’s the result of subtle cultural ingraining, it is not conscious. It’s easy to absorb from our fucked-up media that white straight men are the “neutral,” the one that anybody can identify with and step into the place of, something we often want for our protagonists. That doesn’t make it any better, but I think it does mean those people are more likely to be open to learning better and working in the future not to make the same mistakes.
So in many other cases, I tend to think the biased treatment of non-protagonist characters comes not necessarily from a disbelief in the inherent humanity of women or racial and sexual minorities. It’s a side effect of having most of our protagonists be straight white men.
When you write a story, everything tends to spin out from the protagonist. People rarely start from circumstances and then design a person to center things around. When you end up with a straight white male hero, everything else becomes designed in relation to him. Because white people tend to come from white families, his parents and siblings are probably white too, so if you include them, you get more white people. Because straight people form romantic attachments to people of the opposite gender, his love interest has to be a woman. Because you already have a protagonist, even if you DO decide to include more diverse characters in addition, they HAVE to be subordinate in importance to the story because the most important role is already filled. It’s not necessarily that this theoretical writer don’t think people of color, queer people, or non-male people are interesting or that they’re unwilling to write about them— it’s that some of the logical consequences of the white straight male protagonist screw everything up.
Then, moving on from there, I can also see why those secondary characters end up getting killed or otherwise imperiled for the sake of the story. In drama, the stakes must be high. The protagonist must have a strong, compelling motivation to make them go through the maximum amount of struggle and the most intense emotional journey. Often the most powerful and relatable way is to put their loved ones in danger, because it is a fairly universal human experience to want to protect those you love and suffer as they suffer. This means family, friends, romantic interests—who as we have established are dictated in relation to what makes sense for the protagonist —end up on that chopping block for the emotional weight. This gives us fridging and the resultant dehumanization and marginalization.
I saw this a lot in grad school. When somebody else wrote a story led by women, or people of color, or queer people, there was no real trouble on their part to invest in it, to believe that these people could be active or have stories worth telling. But when they told stories pulled from their own brains, they defaulted to white straight dudes, surrounded by other white straight dudes.
Now, I’m not saying that’s okay or that makes it not so bad. Far from it, there is something INTENSELY problematic with the idea that only WSM can be active or heroic or deserve to have their story at the center. But I do believe puzzling out why things happen is valuable, as when you attack the cause you might have a better chance of fixing the issue. And I tend to believe that people default to these WSM protagonists because they’ve been taught that the highest number of people can relate to them, and then all the other problems spin out from there.
So I think we as artists need to make a real effort to have different protagonists. Because every choice you make on them will ripple out into the rest of the story. So changing the center will change every spoke in the wheel. I know when I have even made the simple choice of having my protagonists be women (Mrs. Hawking is a great example) the tropes of the story are flipped just by the female character being the most important actor. If you think about it, it’s a relatively easy fix. We just have to change one character, and it helps the rest fall into place. :-)