We Just Don’t Have the Budget!
You could switch out the word budget with time, experience, or need. Ultimately, it all comes down to a lack of information, which translates into little or no understanding of the business case that exists for ensuring that a Web site is accessible.
If I were to tell you that I know of a relatively untapped market that, in the USA alone, accounts for over 12% of the entire population, what would you say?  This same market has an aggregate income that tops $1 trillion and includes $220 billion in discretionary income.  Would you want to know more? Would you want to know who this group is and how you could serve them?
This market is, of course, people with disabilities (PWD), which has an estimated world-wide population of 1.3 billion people who, together with their friends and family, control over $8 trillion.  That’s a pretty affluent market that just happens to be the size of China! Now that you know this, could you make some room in your budget to ensure that this demographic can access your Web site or buy your product? No? Still need a few more reasons? No problem.
Do You Know That You Could Be Sued for Having an Inaccessible Web Site?
If your Web site discriminates against disabled people by not allowing them to access the content that an able-bodied person could, you are in danger of facing litigation.
In 2006, the National Federation of the Blind (NFB) filed a class-action lawsuit against Target. They claimed that blind people were unable to access much of the information on the defendant’s Website or purchase anything from its Web site without assistance.
Target tried their best to have the case thrown out, but eventually settled for $6 million in 2008. Additionally, the court ordered the company to pay over $3.7 million in legal fees to the plaintiff. Now, I’m not familiar with the infrastructure of the Target Web site, but I feel pretty confident in saying that it wouldn’t have cost them $10 million to raise the level of their Web site’s accessibility. In fact, in 2010, Target and the NFB established a partnership to ensure equal access to blind consumers on Target’s Web site.
I hear what you’re saying, though. “Target is a huge company, and this example is a convenient outlier right?” Yes and no. There have been other examples of lawsuits against large corporations regarding their Web site’s inaccessibility. While this has been the exception rather than the rule, that may all be about to change.
A Wall Street Journal article about a case in which the NFB brought a lawsuit against H&R Block stated: “The Justice Department laid out a set of compliance standards for accessibility on commercial Web sites. … Lawyers say the H&R Block case offers a clear road map for disability-access claims and is likely to spark a surge of lawsuits with violations that pertain to Web sites.” 
In response to this issue, Adam Saravay, a partner at McCarter & English, said his firm was seeing a sharp increase in disability-access lawsuits over physical barriers. He went on to recommend that Web site owners speak with their developers about updating their Web sites to meet accessibility standards.