I just read a really good post at PM Hut titled Agile Myths Debunked. Sanjeev Singh listed 12 reasons people say Agile won't work on their projects and how they are misinformed. His list included:Indiscipline, lack of planning, no documentation, no QA involvement, not for fixed bid projects... and the list goes on.
From my experience, organizations commonly get caught up in their project management processes. I think they fail to realize, when saying they can not use Agile, the goal is to deliver value to the customer. As professionals, we should focus more on how it can work and less on why it can not. I do believe Sanjeev's listed myths are the primary reasons Agile isn’t more broadly adopted in some markets. I’ve sat in meetings and heard those same myths listed as though being read directly from his blog post. Only by educating clients and debunking these myths will we see more adoption. I'm not saying Agile will work in every circumstance for every project. What I am saying is you should never discount something unless you've at least tried. Do your research and see where you can make it work.