All Presidents since Nixon have released their taxes and I’m not running for President
IOW, it's a tradition that he, for his own reasons, doesn't care to continue, and he doesn't have to. You could ask why it took Obama so long to release his birth certificate, and why he only did it when it became a problem. You could ask why Obama refused to release his school records. You could ask why Hillary wouldn't release her medical records, and on it goes. People still have the right to privacy, even if they run for president. Naturally, the haters and usual suspects jump on whatever is withheld as containing the smoking gun that will allow them to finally "get" whoever it is they're mad at.
There's a law mandating tax returns be turned over to specific House committees when requested. Trump is ordering his staff to ignore the law.
Not really accurate. There are specific committes who can request tax information from the irs only during closed executive session. This means there are confidentiality rules that apply. Only those on the committee can view the information, and whatever they see cannot be shared outside of that closed executive session.
Also, the information requested cannot be information that identifies a particular person.
There has to be a specific reason for requesting the information. In other words, they have to basically know what it is they are looking for, and use the documents to confirm it. They are not allowed to request the documents just for the purpose of seeing what is in there to see if they can find any wrongdoing.