Now that post-invasion Iraq has moved towards being a (problematic) democracy, there's a vacancy in the Axis of Evil. North Korea and Iran have a firm lock on their spots. There are a number of contenders including Yemen, Somalia, Lebanon, but none have the credentials of support for terror and belligerence that Syria does.
And you have to have mutual cooperation to be part of an axis. Syria's hereditary president, Bashar al-Assad, reinforced that on a recent visit to Iran, where he boasted of the two countries' "exceptional ties," probably referring to their mutual cooperation in establishing, arming, and assisting the terror group Hezbollah in Lebanon, which acts at the behest of Damascus and Tehran.