What significance could the information in Donald Trump's tax return have to his campaign? addresses what information could be gleaned from Donald Trump's tax returns if he allowed them to be released. But I wonder why there's so much focus on the tax returns, rather than other financial records.
While there's certainly some useful information there, and you can get a rough picture of a person's overall financial situation, it's still missing quite a bit. A person or corporation's tax return only reports those aspects of their finances that affect their taxes. For instance, you only have to report investments that produced income that year, as well as investments that you've sold so you can report capital gains and losses.
Why not demand access to candidates' more detailed financial records (e.g. net worth statements, budgets, corporate balance sheets)? Is it simply because there's no requirement that individuals keep detailed personal records like this, but tax returns are required, and falsifying them is illegal? But most wealthy people do have accountants and money managers who track their finances, so the records most likely exist and they could be requested. And if they have much of their wealth in businesses, like Trump does, those corporations will have detailed books.
Or is it even simpler: is there no law authorizing Congress to demand this information? And in the current political climate, it would be impossible to pass such a law -- the Republican Senate would not vote for it, and even if it did manage to pass, it wouldn't be by a veto-proof super-majority. On the other hand, there's a 1924 law that allows some congressional committees to obtain the tax returns of any taxpayer, so they go with what they can legally get (Trump is challenging that law, and it will probably end up in the Supreme Court). And even this law doesn't allow the returns to be disclosed publicly, unless the taxpayer consents.