As stated earlier, we cannot say exactly how Google decides to accept or reject a site. However, one thing is for sure - the main criterion for approval is always the content of the website!
Most sites get rejected simply because they do not have any - or enough - good content. What constitutes "good" content? We can say what it is not. The types of websites that only have links and "filler" content aimed at attracting Search Engines, and sites that do not offer any informational content but solely focus on their own products and services. Google is one of the smartest companies on the Internet, and they can easily tell "junk" content from "real" content, even if many web surfers themselves can't.
Google approves websites that have significant real content. These could be in the form of informational articles, analyses of various topics, and much more. For business websites that solely focus on selling their products and services through their website, including informational articles about their industry is smart, and This data is usually pretty easily available.
Another reason why most business websites do not qualify for the program, is that most businesses are specialized, and these websites are most likely to get ads from their competitors, as only those ads would be relevant. As discussed, competitor ads would be blocked by you, filtered by Google, or both, so an AdSense program that serves no ads makes no sense!
Similarly, websites that focus only on links for generating search engine traffic do not have any content at all. According to both common sense and Google, these sites may rack up fairly high traffic scores and so forth, but they do not qualify for AdSense.
One of the best and simplest strategies is to include at least 30 to 40 informational articles of 400 to 450 words each and update them from time to time. Writing these articles yourself may be a daunting task. However, the good news is that there are considerable websites that offer articles for free, and plenty of professional writing resources who can not only write high quality material for you, but can especially optimize that material to work well with search engines and the "qualification" processes for systems like AdSense.