Mostly because Russia and China are on Irans side and if we enter, they'll likely come to Iran's aid turning the whole thing into WWIII with strong nuclear possibilities.
The problem here is the world has speculated as to whether or not Iran already has nukes. Yesterday Iran made the following cryptic comment: "a great surprise will occur - one that the world will remember for centuries". This sort of comment hints at them having and intending to use nuclear weapons. That, or (and I believe we've all heard the rumors) they actually do have terrorist cells within the United States and plan to activate them.
Iran is radical and capable of doing quite a bit of damage if either of the two possibilities mentioned above have validity, and then there's Russia and China again too. Now, there are reports that Iran's missile supplies are running low. The Ayatollah's regime is getting desperate as they are losing power. If they have enough enriched material to make a nuclear weapon, they will use it.
My opinion is we give Israel a couple of MOAB bunker busters to take out the Iranian Fordow nuclear plant (or whatever munitions are capable of taking out this hardened site) and sit back. We aggress only in direct response to actions against us and our asset's. Hopefully, that will keep Russia and China out of it.