We all like to see ourselves in the books we read and the movies we watch. And if you're a straight, white, able-bodied, cisgender person with no serious mental or physical illness then you're just fine. You're everywhere.
If you've got anything about you that makes a character difficult or unusual, then you're basically screwed. Gay? You might get to be the Gay Best Friend if you're lucky. Bisexual, pansexual, or asexual? Forget it. No one, apparently, wants to write about you. Black, Latina, or Asian? No one, apparently, wants to write about or cast you. Transgender, genderfluid, or agender? No one, apparently, wants to write about you either. Unless the story is all about your Unusual Gender Alignment. Disabled or sick? Apparently it's no fun to read about someone who maybe can't do quite everything an able-bodied or healthy person can do.
Well I can tell you it's fun for the person reading. If you're the kind of person who's commonly written about then you can't possibly understand the blinding, heady relief of finding a character who's just like you. A month or so ago, I picked up a book called Fangirl by Rainbow Rowell. I'd heard good things about it. I thought I'd give it a try.
The main character has social anxiety. Do you know how incredible it was for me to realise that characters could have that? I'm a writer and even I've never given my characters social anxiety because I never thought that someone with it could be the heroine of a story. Every decision that character made was something I would have done, and she was influenced by the same thing that plagues me every single day. I couldn't believe that she could have that illness and still manage to be the heroine of a story.
She made me believe I could do that too.
When you write or cast yet another straight, white, able-bodied, cisgender person, you're telling everyone who doesn't fit into those criteria that you don't want to write a story about them. That you can't be bothered stepping out of your comfort zone to give them a character who's just like them. You're telling them that they're not worth being the hero/heroine of their story. And maybe you're not doing that intentionally. Probably you aren't. But that's the message they get anyway.
It will only cost you a little bit of effort to drastically improve your writing and to make someone else's day.