"There is a simple answer"

[starroute]

2. The longer Mitt strings this out, the more possible or actual dirt people will come up with

If the first-order problem with Mitt's tax returns is that they show him as even richer than we knew and paying even less taxes, he might have released them and ridden that out.

But by stonewalling this way, he raises the question, "What in them is so bad that he would risk everything to conceal it?"

And that opens the gates for every kind of potential crime and conflict of interests to come pouring out. They may not all be true, but people are speculating about them and looking for evidence of them. And things just get worse and worse.