PDF/A isn't necessarily accessible. First, PDF is very hard to impossible to scale for people who need big writing, due to reflowing being unavailable or not working properly. Text in PDF/A can still be a big image, e.g. in a scan for archival purposes, if you are "lucky" there is an OCR overlay, but then your screenreader reads OCR'd text which is hit-or-miss. And text ordering is supposed to be "reading order" in PDF/A, but many tools, especially DTP tools make a mess of it and jumble up the (screenreader-visible) order of text in textboxes, columns and sometimes even paragraphs.
IME PDF/A is better than nothing, but far worse than plain HTML for accessibility..
Scanned images are totally PDF/A-compliant. The standard is just about reproducible archiving, nothing else. Bitmaps are perfectly reproducible and pass every compliance check. That PDF/A is more accessible than any old PDF is just a side-effect.
IME PDF/A is better than nothing, but far worse than plain HTML for accessibility..