Accessibility is the law. Several landmark pieces of legislation were passed during the 1990's that enforce the concept of "access for everyone", including access to information technology. These included the Americans with Disabilities Act (ADA) of 1990, the Telecommunications Act of 1996, and the Rehabilitation Act Amendments (Section 508) of 1998.
Accessibility is the business savvy thing to do. An accessible website tends to be more usable and better optimized for search engines than an inaccessible site. This helps people to find and use your site more easily. An inaccessible site is like a locked door on your web store. You might be turning away more potential customers than you think. Accessible sites benefit not only people with disabilities, but also people who use less common web browsers, have slower Internet connections, use older computer hardware, or access the web on their cell phone. Accessible sites provide more people an opportunity to enjoy what you have to offer on the Web.
Accessibility is the right thing to do. There are millions of people with disabilities. Types of disabilities include: visual, cognitive, mobility, speech, and hearing. People may possess one disability or a combination of disabilties, all with different degrees of severity. Disability comes in many forms, not all of which are hereditary, long term, or severe. Some can be caused by accidents. Some can be caused by aging. Some can be temporary, such as misplacing your reading glasses. (Just try reading 8 point size font without them!) Technology can and should be a great enabler for all people, regardless of ability.
Not all people are born "disabled", but anyone can become disabled, even temporarily.