The Wall Street Journal's Bret Stephens unearths numerous explanations for the Obama administration's approach to Iran:
In sum, the conclusion among Israelis is that the Obama administration won't lift a finger to stop Iran, much less will the "international community." So Israel has pursued a different strategy, in effect seeking to goad the U.S. into stopping, or at least delaying, an Israeli attack by imposing stiff sanctions and perhaps even launching military strikes of its own.
Thus, unlike Israel's air strike against Iraq's reactor in 1981 or Syria's in 2007, both of which were planned in the utmost secrecy, the Israelis have gone out of their way to advertise their fears, purposes and capabilities. They have sent warships through the Suez Canal in broad daylight and conducted widely publicized air-combat exercises at long range. They have also been unusually forthcoming in their briefings with reporters, expressing confidence at every turn that Israel can get the job done.
The problem, however, is that the administration isn't taking the bait, and one has to wonder why. Perhaps it thinks its diplomacy will work, or that it has the luxury of time, or that it can talk the Israelis out of attacking. Alternatively, it might actually want Israel to attack without inviting the perception that it has colluded with it. Or maybe it isn't really paying attention.
Or maybe, the Obama administration believes it is not in America's interest to embroil itself in a third war in the region and that an Iranian bomb, while undesirable, is not an "existential" threat to the United States. Maybe the administration appreciates that Israel and the United States are two different countries, with different interests, risk-tolerances and priorities.