Why did Wikipedia move from GFDL to Creative Commons?
A core piece of what makes Wikipedia and similar broadly collaborative projects work is the concept of a free content license: an explicit agreement by every contributor to forego many of their rights as copyright holders, and permit widespread reuse with few requirements beyond simple attribution.
When Wikipedia was established, the GNU Free Documentation License (GFDL) was the only applicable and widespread free license. The GFDL was initially designed specifically for software manuals, and in some ways was not ideal or practical for a project like Wikipedia, which hadn’t been envisioned when it was created.
But Creative Commons built a collection of licenses intended for a much broader range of kinds of content.
It was also covered and discussed on Slashdot a couple times:
And here’s the Wikimedia page devoted to conducting the vote and explaining the reasoning and the process.