“That was a decision made by Truman, and followed by Eisenhower, Kennedy, Johnson, Nixon, Ford, Carter, Reagan, Bush I, Clinton, Bush II and Obama. That is why the U.S. Embassy is, and always has been, in Tel Aviv.”
This was initially an oil decision. Since, it has been for political correctness. But, as the world becomes educated that the religion of peace isn’t the pendulum will swing the other way. The finality will be when the US is oil independent. Then, Jerusalem will be Israel’s capital and our embassy will be there.
Ostensibly, the original reason was that the United Nations Resolution in 1947 that called for the partition of British Palestine into a Jewish and an Arab state called for Jerusalem (and Bethlehem) to be an "international" area, owned by no country and administered by the U.N. Neither Israel (which seized West Jerusalem in the 1948 war, and the rest of it in 1967), nor Jordan (which seized East Jerusalem and Bethlehem in 1948 and lost both of them in 1967), ever accepted this condition, but President Truman used it as a basis to decline to accept Israeli sovereignty over Jerusalem, and no President has ever changed that policy.