It’s the law! In 1998, Congress amended the Rehabilitation Act of 1973 to require Federal agencies to make their electronic and information technology accessible to people with disabilities. This law applies to all Federal agencies, some state and local agencies as well as any entity creating websites for applicable agencies.
There is also a strong business case for accessibility. Studies show that accessible websites have better search results, reduced maintenance costs, and increased audience reach. All of which increases profit margin for a business.