Outside of business specifically oriented towards people with accessibility issues, the energy just doesn't translate into VC.
Companies who do try to shoehorn it in when products are more mature usually have:
A codebase with a frustrating amount of refactoring in order to retroactively get things in line.
Development inertia where it's seen as a low value activity among developers and product owners
Lack of clear guidance/tools/processes to QA new work
Lack of will to retroactively identify the breadth and scope of changes you even want to make
There is no mystery. It's not going to get you sexy VC money at the beginning, and then it's bizarrely more work than you'd think once your project is sufficiently large.