25% of the world's crude oil was shipped out of the Gulf at that time. The Iran-Iraq war had caused the prices of crude to skyrocket as many insurance companies refused to cover any shipping in the region. So the US felt compelled to protect the exports of crude oil from "friendly" nations. While continuing to blockade Iranian imports/exports.

For more details on the political situation at that time, I recommend reading The Persian Puzzle by Pollack.

https://www.amazon.com/Persian-Puzzle-Conflict-Between-Ameri...

Additionally, to help explain how messed up the shoot-down was, Sources of Power by Klein.

https://www.amazon.com/Sources-Power-20th-Anniversary-Decisi...