It's easy to set up a wiki installation on a web site.
It's hard - much more difficult than you probably think - to take the next steps of:
- protecting from spam and vandalism
- continuing to develop the site
- finding content
- creating community
- managing the content for quality and structure
These things take a huge amount of collective work.
It is a rare community that has even one obsessive wiki admin/contributor, let alone the half dozen or more that are needed to make a successful wiki.
Think globally - collaborate don't compete
Collaboration and competition are both powerful creative forces in nature and society. They each have their place - but the nature of wikis is radical collaboration.
See Permaculture wiki #Think globally: A call to collaboration
See also Green wiki, Appropriate technology wiki and Wiki synergy.