Who was the last person to be tried for treason in the United States ; and why are treasonous acts ignored ?

The easiest answer to that question is that treason is the only crime defined by the Constitution of the United States of America (Article III, to be precise), and that anything, no matter how similar to the perceived definition of the term, that falls outside of those guidelines cannot be tried as treason. As a result, such trials are extremely rare. Instead, people are often charged with related offenses, such as espionage as in the case of Julius and Ethel Rosenberg, or plead guilty to lesser offenses such as John Walker Lindh.