There may be many barriers to advocacy. Of course, it is a broad question, so I will simply give you some basic answers.
Advocacy can cause many problems for people. There is a fear of rejection, ridicule, personal injury, or perhaps even death. Some may not be interested in advocacy if the issue does not affect him or his loved ones directly.
Examples include civil rights, gay marriage, AIDS research, changes in school policies, changes in law, and domestic violence. In many situations, advocacy can cause a person to lose friends, family members or jobs. In some circumstances, advocacy can cost a person his life.