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.