Likely because there isn't a good reason.
Think of the Saudi oil field as a surrogate for a US warship - only the ship can be moved.
That's the message from Iran, get out of my pond.
They have demonstrated a way to deliver a lethal blow without leaving a clear trail.
Based on performance of the drones, it's doubtful they came from inside Yemen.
Even if they did, even the UN is on record as saying Iran supplied the weapons to the rebels.
What Iran is counting on is world and US political pressure to keep the US (Trump) under wraps.
They don't want to get into a shooting war they are sure to lose and lose quickly.
They got caught with the mine, but tried to deny it. A hell of a lot of people bought their story, even with the surveillance video.
So now they have a way to sting the US Navy and walk away without getting even a bloody nose.
Message, lift the sanctions, let us develop our atomic bomb and export it to terrorist groups all over the world so we can wage jihad.