I put it to colleagues like this:
Our product is sold all over the world. We have no way of knowing who will be using it. So we should make the product as accessible as we can, because at some point, a potential client may have a choice between our product and a competitor’s, and if the only difference between the two is that they have a member of staff with particular access needs who finds the competitor’s product easier to use than ours, then we lose the sale.
Many companies have hard-nosed business types in the management team who will be immune to the argument that “It’s the right thing to do” and say “What’s in it for us?”. I found this argument works, for some strange reason.