Free software has tended to serve the leading edge of the computing community - hackers, etc. - first. General users have tended to follow later, and those with access problems after that. That allowed Microsoft to use the relatively poor support for these communities as a stick with which to beat ODF during the early stages of the ODf vs. OOXML battle in Massachusetts. Things have moved on, but it remains true that free software's support for all users, including those with disabilities, has lagged somewhat behind proprietary offerings.
On Open Enterprise blog.