Because compared to previous time periods, now the US actively interfered in what was happening in Europe as well as around other places in the world. This was the main difference that distinguised isolationism from intervention.
Faced with economical hardship, the United States decided that it would remain away from the war on fascists at all costs.
Ultimately, the fear from fascism triggered the Americans to support putting an end to that "evil" in the other side of the world before it could even reach them.
