There are some basic age and date criteria the site uses as a starting point. Beyond that, things become more subjective, but the goal is to include people that are as widely known as possible. The greater the name recognition a person has among ordinary people, the more likely he or she is to be included. The site isn't concerned with "important" people, only "famous" people, and those two things aren't necessarily identical. Plenty of important people are virtually unknown to the general public and plenty of famous people haven't made any particularly noteworthy contributions to society. The site favors famous people over important people.
Another possible reason is that the person is not judged famous enough to be included. There are tens of thousands of people that have done something that was in the public eye in some way. The goal of the Dead or Alive? site is to list the most famous among them. The site has no pretensions of being comprehensive in any one particular area. If you are looking for obscure movie actors, or writers, or sports figures, or everyone who ever played with a certain band, this website is not for you. Check the Related Links page for more help. It lists web sites that specialize in comprehensive coverage of various fields of endeavor that might very well include the person you are looking for.
Finally, remember that all name submissions to the site are treated as suggestions. There is no guarantee, implicit or explicit, that any particular name will be added to the site.