Perhaps you are unaware of Iran's history with the USA. I took this snippet from wikipedia, it should serve as a little primer:
Mohammad Reza lost support from the
Shi'a clergy of Iran and the working class due to alleged corruption related to himself and the royal family, suppression of political dissent via Iran's
intelligence agency,
SAVAK (including the arrest of up to 3,200 political prisoners),
widespread torture and imprisonment of political dissidents,
[8] banishment of the
Tudeh Party, U.S. and UK support for his regime, his modernization policies,
laïcité or secularism, conflict with wealthy merchants known as
bazaaris,
relations with Israel, and clashes with leftists and Islamists.
US/UK power brokers supported his brutal regime, as they have supported such regimes in other countries. US/UK also backed Iraq's war against Iran. Those would be a few good reasons.
A third reason is the US is often sabre rattling against Iran ("Axis of Evil"). Whatever reason is given for this, I suspect the real reason is that Iran is one of the few holdout countries, not having a Rothschild owned central bank. That actually makes the country more independent in certain ways than so called "free" countries, who have the majority of their media and industry owned (through corporations) by big banks.