Why only insanity as an affirmative defense under 18 USC?

Under 18 USC, only insanity is an affirmative defense. In state criminal codes, there are other affirmative defenses, including "general justification", "justifiable force" and "self defense". Why do these additional affirmative defenses not exist in 18 USC as well?

Answers:
A layman's opinion. Especially with jury trials, insanity pleas are a very special case. The whole subject challenges what we may consider to be explained as common sense.
I think Shannon vs. U.S. exhibits some of the problems.
As a $17 per day juror in my county, I would be confident in deciding legal vs. illegal but I would want specific law for an insanity case.


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