OpenStreetMap’s relicensing (from CC-BY-SA to the Open Database Licence) involved more contributors and, unlike Wikipedia’s, wasn’t assisted by a licence hack.
Wikipedia was licensed under GFDL 1.2 or later, so relicensed by convincing the FSF to release an updated version of the GFDL (version 1.3) that adds a very narrowly worded clause allowing relicensing that pretty much only applied to Wikipedia.
> An MMC [Massive Multiauthor Collaboration Site] is "eligible for relicensing" if it is licensed under this License, and if all works that were first published under this License somewhere other than this MMC, and subsequently incorporated in whole or in part into the MMC, (1) had no cover texts or invariant sections, and (2) were thus incorporated prior to November 1, 2008.
> The operator of an MMC Site may republish an MMC contained in the site under CC-BY-SA on the same site at any time before August 1, 2009, provided the MMC is eligible for relicensing.
Its not totally different - cc-by-sa is pretty similar in spirit to gfdl with no cover text, non-derrivative sections etc.
Of course practically speaking, being able to legally print out a wikipedia article without printing out the entire gfdl is a pretty big difference. To the point where i am aware of people licensing photos under gfdl to claim its open source while in practise its impossoble to reuse practically.
I mean your point still stands that its weird to release a new license just for wikipedia.
It didn't only apply to Wikipedia, there were plenty of other large public wikis that used to be GFDL-licensed only and were able to transition to CC-by-sa.