I think it's down to the fact that most 3rd party accessibility tools are written for Windows/MS Office. This is a result of Microsoft having the greatest market share, rather than any specific features that MS Office has over OpenOffice.
Still, if the lack of accessibility tools is a barrier to increased market share, then it's down to the Open Source community to provide them. And this doesn't have to mean unpaid coders - these tools could come from Sun, Novell, IBM etc.