Bernie's tax returns. Hillary's transcripts. Honest discussion, why is it such a struggle to require transparency from a potential POTUS.

NOTE: There are other threads such as this one. It spirals in a back and forth of Candidate A will release them when Candidate B releases them. We are not having that discussion here. I don't care who goes first or who should have to go first. Do not bring it up here.

So honestly, what reasons are most likely for Sanders to delay the release of his tax returns, and what are likely reasons for Hillary to not release her transcript speeches?

And this goes for both candidates, why, if you do, find this acceptable?