I think, imo, because it is politically motivated. For an example:
Someone Leaked 10 Years Of Trump’s Tax Info To The New York Times
So, already leaked (without proper authorization) 10 years of tax information that was suppose to be private and used for strictly political reasons... a weapon.
How many private citizens would be upset if the IRS began using private information as a weapon? The law says it is private unless there is foul play... if 10 years has already been leaked... what more is there needed?
We know he already is giving all of his salary back to the government.. what reason is there for more tax info?
I always go back to "whatever liberty you give away, you never can get it back". I am more worried about what that means for the average person (precedent) than what Trump did with his money (since no evil has been unearthed).
Now, if we had seen he was a tax crook, then we have legal recourse to ask for tax information. At this point, all we see is that he uses legal loopholes (that both parties use and were created by) - just like we use every legal loophole that people can use.