Doing my own reading and reasoning about releasing technical content  under a free license, I have discovered there are these general approaches:
- CC-BY-SA only (e.g. StackExchange)
- FDL only (e.g. Free Software Directory) 
- both CC-BY-SA & FDL (e.g. Wikipedia)
Choosing one of the "only" streets means your work can only be used in similarly licensed works. Choose the "&" option means your work can be used in any of the three.
Has SE explained why they picked CC-BY-SA only, rather than dual-licensing? (This is a question, not a challenge or a suggestion.)
 Including software manuals, but not code , but including cooking and golfing and maths and stuff cause that's all technical right? :D
 The Free Documentation License is a copyleft content license like CC-BY-SA, but older (conceived at the same time as the more famous GPL license for code). Unfortunately you can't freely copy content between FDL and CC-BY-SA works, so the current solution (if you want to make that possible) is to dual-license. (There is a more elaborate backstory  here, and as far as I understand the authors of both licenses would like to see a day when they are compatible, it's just not currently possible.)
 The inevitable code snippet licensing question, which I finally found an official SE position on.
 There appear to be fan clubs in fiercely in favour of particular approaches, and opposed to others. I hope this question doesn't trigger those sorts of responses, I really just want to know why SE chose not to take the dual-licensing route. A suggestion to change would be a separate post.