One likely reason Iran was willing to sign a rock solid treaty, including inspections, stating it would not pursue a nuclear weapon, with Obama(US), China, Russia, France, Germany, and the UK is probably because Obama is an honest man and could be trusted, plus, the whole world was watching. Donald Trump has no allies mainly because he can’t be trusted and doesn’t work with others, plus he ripped up the old treaty. Therefore, there are no mechanisms in his proposed treaty to keep him to his word. This makes Iran’s trust level practically nonexistent and level of caution extremely high.