OSX also had accessibility support built in from the beginning. Apple made it a big deal that every computer should be installed with it by default, because it used to be the case in Windows that you had to install support manually, and only one computer in a classroom in the corner would have support. The idea was any child should be able to use any computer.
OSX has been generally good for accessibility for a second reason. Besides user-facing features, Apple's core APIs like Cocoa have accessibility built into to the widgets. When you use standard Cocoa controls from buttons to textviews, Apple already provides useful behaviors for accessibility in them. That way, even if a developer is completely oblivious to accessibility needs, as long as the developer was being a good Mac/iOS citizen, their program automatically inherits accessibility support.
This is where most 3rd party toolkits completely fall down. Cross-platform toolkits, 3rd party web browsers, video games... They reinvent all the GUIs/widgets themselves, *poorly*, and they always miss this aspect. And very few call them on it.