In fact one of the main reasons people with autism and learning disabilities, in particular, are unable to vote is due to a lack of accessible information and material. The National Autistic Society has also heard from people with Asperger syndrome who have experienced a great deal of anxiety and difficulty with managing the paperwork required. These issues affect everything from how they join the electoral register, and the practicalities of how they cast their vote, to their ability to make an informed choice about who to vote for. People with learning disabilities say they do not understand information on policies or what candidates and parties stand for - and they are not the only ones. A recent survey by Mencap showed that 90% of the voting population does not understand phrases in political literature.
This article is specifically about Britain but points to some similar problems in the United States.