People have specifically challenged / questioned his net worth, then advocated for tax returns as if that would answer that question. I've read such in forums and I've seen talking heads on TV express it. I didn't realize so many people have never done their own taxes before.
OK, so specifically talking about where his past wealth came from- I have some issue with that as it occurred as a private citizen before he took public office. Anything since should be on the table, or perhaps as far back as his official announcement of candidacy, but prior to shouldn't be pertinent. This country has recently gone down the path that any nefarious event that happened in any point of a private persons life, even decades & decades earlier, should 'rightfully' prevent that person from earning a living in the present or future. Too commonly; it amounts to a vindictive overreach. Or 'vigilante social media vengeance', if you like.
surreal : What specifically would -in an ideal situation- you expect to learn from his personal tax return? As dwight alluded to; you're not going to see -for example- a 1099 from the Kremlin. There's just not much information on them as to sources of income. And dwight is correct; if it was illicit, you're not going to see it at all.
But dwight- they don't teach tax code in K-12, bud.
Until personal tax report release becomes a legal condition of the office, as opposed to voluntary as it is now, I would defer to the decision of the candidate. That said, if he clearly said he would release them (which he did, but only did what- 1 year?), then he should uphold that 'promise'…. but I never hold my breath waiting.
Interesting accounting of the Trump Organization; it is bewilderingly multifaceted :
https://en.wikipedia.org/wiki/The_Trump_Organization