In principle, it would be possible to write Tricki articles and put them on Wikipedia. So why are we not doing that? Perhaps the main reason is that we want a typical Tricki to be different in an important respect from a typical mathematical Wikipedia article: it will be focused on methods rather than on subject matter. For example, an article explaining what a Banach space is belongs on Wikipedia, whereas an article giving methods for proving that a norm is complete belongs on the Tricki. Of course, we expect there to be many links from Tricki articles to Wikipedia articles (and indeed there is a formatting feature that makes it particularly easy to insert such links). If the Tricki is a success, we hope that there will be many links in the opposite direction as well.
Even so, one might ask, why should an article on completeness proofs not be added to Wikipedia? Again, it would in theory be possible to do that, but we feel that it is better to have a separate site where it is a major priority to address the navigation difficulties that such articles present. And there is also something to be said for drawing a clear distinction between the techniques-based articles on the Tricki and the subject-matter-based articles on Wikipedia.
In addition, having a separate site has allowed us to introduce features that are designed to be particularly convenient to mathematicians. For example, it is much easier to write mathematical symbols on the Tricki than it is on Wikipedia if you are used to TeX and LaTeX. There is also a hidden-text feature that makes it possible to signal to the reader that a fuller explanation is available if needed. This means that one can write an article without stopping to give basic definitions or explain easy proofs, but one can also include these basic definitions and easy proofs in a hidden form that can be revealed at the click of a mouse by those who would like them. This feature, which Wikipedia does not have, makes it possible to write an article that can be read comfortably by mathematicians of widely differing experience.
More reasons?
How do I create a Tricki article?
How do I make my article show up in searches?
What is a Tricki article allowed to be about?
How do I edit or comment on an existing article?
Tricki