Why Wikitravel isn't GFDL
The GFDL was developed to support making Free Content versions of software manuals, textbooks, and other large references. It has some heavy requirements for what you have to distribute with a document under the GFDL. For example, you have to include a copy of the GFDL, a changelog, as well as "transparent" (i.e. source) versions if you distribute over 100 copies.
These requirements aren't really all that onerous for large volumes of text. But for Wikitravel, we really want to have each article redistributable on its own. Wikitravel articles can be as small as 1-2 printed pages. For such small documents, it just doesn't make sense to require people to pass out another 10 pages of legalese text, as well as floppy disks or CDs full of Wiki markup.
Consider these small "publishers" who would distribute stacks of photocopied printouts of Wikitravel articles:
Burdening these publishers with restrictions meant for software documentation or textbooks would mean that they'd either ignore our license – a bad precedent to set – or, more likely, just not use our work.
We make our content Free so we can collaborate on this wiki, but also because we want it to be seen and used. We can't serve travellers with useful information if they can't get to that information in the first place.
A lightweight alternative
The license we've chosen, the Creative Commons Attribution-ShareAlike 1.0, is much easier and more lightweight. We think that using the Attribution-ShareAlike 1.0 license (by-sa) meets our goal of having copyleft protection on Wikitravel content, without putting an excessive burden on small publishers. All that needs to be included are copyright notices, author attribution, and the URL of the license; this can be done in a short paragraph at the end of the article.
The big downside of not using the GFDL is that GFDL content cannot be included in Wikitravel articles. This is a restriction of the GFDL – you're not allowed to change the license for the content, unless you're the original copyright holder. This is kind of a pain for contributors, but we figured it was better to make it easy for users and distributors to comply with our license.
Creative Commons has plans to issue a new revision of their suite of licenses at some point in the future. Compatibility with other Free licenses is "a top priority", and we can expect that some time after that version change, articles created on Wikitravel can be distributed under the GFDL. (GFDL compatibility was supposed to be part of the 2.0 version change, but didn't make it in.)
If and when the CC licenses become GFDL compatible, other Free Content authors can include Wikitravel content into their work, even though we can't include GFDL work into Wikitravel.
If not having your contributions under the GFDL is unacceptable to you for some reason, and you can't wait the few months until the new version of the by-sa comes out that would be compatible with the GFDL, there are a couple of different options.