To answer why now, I have a theory. (I apologize beforehand for any ignorance, I only know a very minimal history of US - Iran)

1. Government. The US has wanted to control Iran from a long time, and succeded once in 1953. When the revolution of 1979 happened, they lost their hold in Iran’s goverment. They have constantly been trying to topple Khamenei’s regime since then. Khamenei was getting old, and if he names a successor and they come to power, the current regime system will be cemented and more difficult to overthrow from outside.

2. Oil. After the Russio-Ukraine war started, America has been trying to stop countries from buying Russian oil and make them buy crude oil from American companies. The US needs to get hold of two things now - Oil wells (Venezuela, Arabia, Iran, I think Palestine too, I’m not sure) and trade routes. Iran has control over the Strait of Hormuz , which transports about 1/4th of world crude oil trade.

3. Dollar. Both Maduro and Khamenei were trying to change the currency of oil trade, that is the American dollar.(This goes back to some agreement made between USA and Saudi Arabia)