They supported the Iranian revolution expecting they will be able to control the outcome. That was ending the British relationship and starting an only American one with their oil.

This worked out pretty well in Saudi Arabia, which was very primitive and had very small population.

In the end,though,in Iran they are Muslims first and Iranians second, with a long History. They could have accepted all the money from infidels that they could get, and promised whatever to them, but their own religion says they could(and should) lie to infidels. So they did it.