It’s different objectives. Russia is interested in taking territory and inciting fear. Ukraine is interested in self defense and instilling war fatigue. Both of them need people to feel unsafe and vulnerable, and to actually destroy soldiers and hardware.

The US, on paper, wants to prevent Iran from having a nuclear weapon. Realistically they want to make a show of force for internal politics.
Neither of those requires radioactive material to be hit by explosives.
Iran needs to show that you can’t bomb them without retaliation. They also need to not anger the country hosting the US military base, and also to not do something that causes the US to strike with significant force.

So the US can announce it’s plans and both make a show for internal reasons and also setback any ambition that Iran has towards a weapon.
Iran has to hit something otherwise it’s an unacceptable show of weakness, but hitting actual US territory would be suicide. Nearby bases are a good choice but require coordination with the host nation. “I’m not shooting at your house, I’m shooting at the person in your living room” is a hard sell to make if you want to keep peace with the homeowner and you don’t give them any warning. Additionally, doing too much actual damage to the base can trigger further retaliation, which you want to avoid.

Also, the US has a pretty comprehensive surveillance system. It’s difficult to move large amounts of material from a place being watched from space without it being followed, even more so if there are drones involved.

Different objectives call for different tactics. Ukraine and Russia need fear and surprise for theirs, and Iran and the US don’t.