Why didn't any president.... Because it is not the president. It's the House mainly. Even when republicans held the House they could agree on anything.
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.
Am I surprised, no. I wouldn't release them either. My taxes, my business. You don't like it, don't vote for me. In any case, Trump's tax returns are irrelevant to the issue. Or are you actually expecting Biden to fix this?
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.) 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.