Most of the reasons boil down to the obvious fact: the US government lies all the time. From Caitlin Johnstone at medium.com:
In a move that surprised exactly zero people, Secretary of State Mike Pompeo has wasted no time scrambling to blame Iran for damage done to two sea vessels in the Gulf of Oman on Thursday, citing exactly zero evidence.
“This assessment is based on intelligence, the weapons used, the level of expertise needed to execute the operation, recent similar Iranian attacks on shipping, and the fact that no proxy group operating in the area has the resources and proficiency to act with such a high-degree of sophistication,” Pompeo told the press in a statement.
“The United States will defend its forces, interests, and stand with our partners and allies to safeguard global commerce and regional stability. And we call upon all nations threatened by Iran’s provocative acts to join us in that endeavor,” Pompeo concluded before hastily shambling off, taking exactly zero questions.