Because we now have a leak of Trump tax information, although it’s fragmentary and from 20 years ago. The Times story suggests–without being able to prove it–that Trump used a loss to wipe out any federal income tax liability for almost 20 years.
A couple of things: 1) There is nothing wrong with using the law to minimize your tax liability–everyone does it, or should; 2) In theory this should open up a populist critique of Trump as the rich guy taking advantage of the rest of us. But the Clinton campaign hasn’t spent much time trying to develop this case, and Trump’s populist branding is so strong it may be that it might not get much footing regardless. The best comeback for the Trump campaign on the tax story is the seemingly counter-intuitive–but bizarrely compelling for many voters–argument that he has exploited the rigged system from the inside so he is best suited to fox it.
All that said, this is obviously not a great story for Trump and the campaign has to hope that Pence can change the post-debate narrative on Tuesday night