Licensing is a pretty big deal for us. Virtually every programmer has some sort of opinion on GPL vs BSD vs public domain vs whatever. Open source projects can live or die because of their license.
Wikis are basically open source projects with natural language instead of code. But I get the impression that few wiki contributors care that much about licenses.
If I had to guess, I'd say that it's because code is more modular and flexible. If you write a useful open source library, the license you put on it can have huge implications for what products end up using it. There's no wiki equivalent to e.g. large quantities of BSD code being used to build a smartphone that sells hundreds of millions of units.