It's the law. The 1990's was a decade devoted to accessibility. Several landmark pieces of legislation were passed 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.
It's the business savvy thing to do. An accessible site tends to be more usable and better optimized for search engines than an inaccessible site. This helps people find and use your site more easily. More importantly, 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 people who use less popular web browsers, have slower Internet connections, use older computer hardware, access the web on their cell phone, or have a physical or mental disability. Accessible sites provide more people an opportunity to enjoy what you have to offer on the Web.
It's the right thing to do. Millions of people are classified as "disabled". 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, as sterotypes suggest. 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!) The Web 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.