Well Trump directly benefits from the current system, promised to fix it, and said fixing it would cost him a fortune. He promised to do these things before he had any record in government. Now he has a record. Now we know it was all a lie. So that's why it's relevant.
I don't think Trump was ever capable of fixing it. Any more than getting Mexico to pay for the wall. If enough people feel this is relevant, then I suppose he won't win.
I don't expect Biden to fix this - as far as I know, he hasn't promised to fix it, like Trump did. (Maybe Biden did promise that, and I missed it.)
What do you expect Biden to do for you?
Part of Trump's 2016 pitch was that he's (1) a savvy businessman who (2) has taken advantage of tax loopholes and (3) he'll fix those loopholes and it will cost him a fortune. Turns out only #2 was true, and that's why he didn't (doesn't) want us to see his tax returns.
Every other presidential candidate has released their tax returns going back to Nixon. And Nixon released his returns under audit and I believe during impeachment or under threat of impeachment.
Trump broke that precedent. Why? Because Trump First. America Second. He's trying to make the Trump family great again and tricked a lot of saps in America into helping him do that.
Yeah, don't really care about any of their tax returns. Maybe you got primo tax returns. I don't. You can judge me by that if you want to. I don't care.
Now that we see his tax returns will see the relevancy to the voters. Blame Trump for a tax system he had nothing to do with at the time or don't. Up to the voters.