The very short answer is :
Wahhabism was an outcast movement centered in the middle of Saudi desert with little or no support.
they got a lucky break when the UK used ben Saud During WWI as a force first against the Ottoman and later against their former ally Sharif Hussein of Hejaz
ben Saud used the wahabies as they were warlike Bedouin warriors and ...
As usual the short answer does not require a lot of explanations
the revolutionary dogma of Iran threatens the US allies in the region (The absolute monarchies of the Gulf states and Israel)
A. The absolute monarchies sees the revolution that toppled the strongest monarch in the area as a threat to their dynasties. they use whatever means they have, to ...
In addition to Colin's excellent list of reasons, there is the Iranian regime's continual incitement against the USA, such as crowds chanting "death to America" at its behest and US flags that people are supposed to walk on to show their hatred of the US. (The linked story talks about people who refuse to walk on the flags).