A "flag of convenience" is when a ship's owner has registered it in a foreign country, in order to take advantage of more less restrictive laws, like safety and tax regulations. Liberia has especially lax laws, and makes it easy for foreign owners to register ships, so that's why it is such a popular flag to carry.
Fun facts for those not aware: this isn't just a reference to the Ugandan warlord character in The Book of Mormon. He's a real guy from Liberia. He was at least as evil as Tayor; probably worse. He killed tens of thousands of people, employed child soldiers, performed human sacrifice to the devil and cannibalized children. He earned his nickname because he literally liked to fight butt naked. He is now a preacher. Not making this up.
Taylor did many of the same things in Sierra Leone before being elected to public office in Liberia.