Check out a contour map of Iran. It is tough to turn a mountain ito a parking lot. Even nukes are linear and can't bend around maountain valleys. Sadam made the mistake of trying to takeout Iran. Even with the financial backing of the USA and Saudi Arabia and the use of chemical weapons, Iraqi forces got turfed out of the low lying floodplains after the Iranians regrouped in the mountains and came screaming back for revenge. I am no trying to be pro-Iranian; just stating the facts.
Not dealing with the facts is exactly how we got into this mess. Don't take a knife to gun fight.
I'm sure Bush thought he would stabilize Iraq before dealing with Iran, and they are also hoping for Iranians to make a change internally. But we are allowing Iran to do too much to us by far.
I'll also note that while Iraq couldn't defeat Iran, the two countries stalemated for years. Iran couldn't defeat Iraq decisively, whereas we did so in short order. Admittedly that was a weakened Iraq.