"why it believes the Iranian government is behind the hacks"
"The company did not go into any detail over why it believes the Iranian government is behind the hacks beyond noting that those targeted included “prominent Iranians living outside Iran.” "
I would imagine other nations, and not just Iran, would also want to spy on prominent Iranians living abroad. Like seeing if they can be mined for useful information that could damage Iran, use them for propaganda, implicate them as spies operating on behalf of Iran, and so on.
It could even be someone else trying to make the attack appear to have come from Iran to remove suspicion from themselves. Or to stoke tensions between the US and Iran. There are a lot of entities that would benefit from continued tensions between the two nations.
Its equally possible that it really is Iran and they got caught trying to open the cookie jar, but it could also be another nation that doesn't want the US and Iran to be on good terms. For instance, Airbus stands to make billions of dollars selling aircraft to Iran, made easier by ensuring that Boeing is locked out of the sale. Saudi Arabia would stand to lose a lot of money if Iranian Oil were to flood the US market. Israel wouldn't mind if the US eliminated / destabilized Iran.
I'm saying that any of those are even realistic possibilities, but just that there are a lot of entities that would be motivated to carry out such an attack.