There are a lot of things that contribute to this. Little value is seen in disabled experiences unless they're focused on the hot issue of the moment is the biggest one because people think disabled experiences are so different and unimaginable. They're not. https://twitter.com/super_crip1994/status/1357784460167688193
There's a lack of mentoring for disabled writers paired with a lack of accessible working environments. Both of these leave disabled people with no way to break into the industry.
And lastly, there's still the thought that accessibility writing doesn't bring a large enough audience to be worth it. CIPT easily disproves that by the audience we've grown in just two years.
Want to add a huge amount of developers to your audience? Add accessibility to your regular content.