Question: Why does a very intelligent person do something that appears stupid?
Answer: Because, contrary to appearances, it is not stupid.
Question: Why does Hillary Clinton, a very intelligent person, refuse to release the transcripts of her Wall Street speeches, even though not doing so hurts her, and is therefore apparently stupid?
Only Possible Answer; Because there is something in the speeches the release of which would hurt her worse than not releasing them.
Question: What is in them?
Speculative Answer: Reassurances to the audience, in some form or other, explicitly or implicitly, that they have nothing really to fear from her as President.
Nothing else makes sense.