Why?
Because The Disabiity Descrimination Act (DDA) specifically mentions accessible websites in sections 7.22 'Provision for people with a hearing disability' and 7.27 'Provision for people with visual impairment'. Section III of the DDA, which refers to accessible websites, came into force on 1st October 1999 and the Code of Practice for this section of the DDA was published on 27th May 2002. There are still many websites which do not conform. This puts the website owners in a vulnerable position with regard to potential legal action.
You can download a copy of the Code of Practice from Equality and Human Rights Commission website Code of Practice