I'm not a historian but its because Reza Shah had denied Brittan and the USSR military access. This was very important as the Allies wanted to send equipment to the USSR via Iran. Churchill called Iran 'the bridge of victory". Its also worth noting that both Russia and the UK have a long history of interfering with Irans domestic affairs, a history summed up as the "great game". In the early 19th century they divided the nation into spheres of influence, though Iran retained territorial integrity. During the Russian civil war Brittan had used Iran as a launch pad to attack the reds, the USSR responding by annexing the northen regions of the nation. It was at this point that Reza Pahlavi overthrew the qajar dynasty in a coup supported by the UK and negotiated the retreat of the Bolsheviks. After this the new Shah Reza Pahlavi tended to try and play the UK and the USSR off one another with support from Germany and the USA, hence why he wanted to keep them from sending equipment through his Country whilst attempting to appease them by offering to expel Germans living in Iran at this time. In general he sought to reduce foreign powers influence in Iran. Unfortunelty for him this policy did fail during the war and he was replaced by his son whom was supported by the United Kingdom.

In the 1940s the main trading partner - if we can call it that - for iran was Great Britain. Though American companies were present prior to the war, the majority of the oil companies were British-controlled. The British, and the Soviet Union (and Russian Empire) had used Iran as a pawn in their competition with each other - what Rudyard Kipling termed "The Great Game" - and foisted lop-sided agreements upon the government of Iran. Some of these "Capitulations" allowed Britain and Russia/The Soviet Union to extract mineral rights, tax free and without hiring local labor in most cases. These mineral rights agreements were not only for ludicrously long periods of time (a 1910 agreement gave the British mineral rights for 100 years), but also were negotiated at gun point (not literally, but usually as part of a peace treaty to end a war, invasion, or incursion).

As you can imagine, England and Russia were not exactly welcome partners at the outbreak of World War II. By contrast,t he Germans had historically been more even-handed and fair in dealing with Muslim countries and the Axis was eager to maintain good relations with the Iranians, especially since it cost the Soviets and British access to oil. It wasn't that iran was refusing to sell oil, but they were nto selling it quite as cheaply as England needed it to be. This, coupled with the somewhat friendly relationship with Germany precipitated the coordinated....'liberation'...of Iran's oil fields in order to more easily prosecute the war.