Wikipedia's content used to be licensed under GFDL 1.2 or any later version. In order to transition to a more appropriate license the Wikimedia Foundation asked

Wikipedia's content used to be licensed under GFDL 1.2 or any later version. In order to transition to a more appropriate license the Wikimedia Foundation asked the FSF to release a new version (GFDL 1.3) that specifically allowed relicensing to CC BY-SA. The new license may be "similar in sprit" but what it allows relicensing to may not be.