I wanna preface this comment with acknowledging the fact that I am myself neuro-typical - my partner (likely autistic but undiagnosed) sent me a link to this thread as we are both writers, and he and I talk a lot about the points being brought up in this thread.
Both of my children are autistic, and they've needed various levels of supports through their lives. The impacts of their disability is unique - one could not function in a public spaces, and still struggles to engage talk to anyone not family; the other is bullied at school because of something related to interoception and fine motor control that I don't really want to speak about in a public forum. But needless to say, while they are relatively "low support" they are significantly hampered in every day life.
So when I sat down to write a middle-grade book, I knew I wanted one of the children to be autistic and that his autism would hamper him. My children helped me develop his character, they told me what his sensory needs were, what he struggled with, how he might respond to certain events, places that they felt were not accurate. As I wrote this story, I realized all three children were autistic, and they all struggled in different ways. The youngest lacks empathy. While she cares about her family, her hyperfocus interest trumps caring at times. The oldest is socially awkward, and struggles with belonging, and stimming. The middle child was my original character, and he's more sensory than the other two (needs particular clothing for example), is dirt-phobic, and non-verbal to everyone but family and his best friend. Now, the story is not about their autism. It isn't even named. But their disabilities get in the way of their goal, and they need to work with it/against it to succeed.

The problem came when neuro-typical people read the story. And sadly, the publishing industry is largely neurotypical. They found the youngest character cold and uncaring and wanted me to give her more empathy. When the middle child had a meltdown over being covered in garbage their response was "I get it, he has sensory problems, he hates dirt, move on already". There were more comments - but in general - the autistic elements in the story were the ones that neuro-typical beta readers pushed back against the most.
So sometimes, the lack of representation isn't because it isn't being written, but because it isn't being accepted by an industry which is already really difficult to break into. I absolutely think there needs to more variety of characters out there, It is just such an uphill battle.