Why would Saudi Arabia want to do something about ISIS if it's cheaper to just invest in deterrence and pay other nations to get their hands dirty?
Besides, Saudi Arabia is a Sunni theocracy. ISIS is a Sunni (or at least Sunni-tolerant, judging by their behaviour) theocratic militia. They aren't predisposed to be enemies. ISIS has no religious reasons to invade Saudi Arabia and as long as ISIS doesn't kill Saudis, Saudi Arabia has no reason to intervene.
Iran is Shia but more concerned with Israel than with killing Muslims. In fact, Shia Muslims tend to be on the receiving end of Muslim intra-faith violence more often than not. The only reason we (the West) don't like them is because they tend to make aggressive moves towards Israel.
There's almost no reason to be concerned with Iran at all (relative to other countries in the region), if you're not Israel. On the other hand, Saudi Arabia is relatively non-hostile towards Israel and makes a great trade partner (especially for weapon exports).
In other words, there is a pattern here: most "terrorist organizations" (ISIS, Al Qaeda, Boko Haram, etc) are Sunni, most of our "allies" in the region (Saudi Arabia, Egypt, etc) are Sunni, most "bad guys" in the region (Iran's Khamenei, Syria's Assad, etc) are Shia. In fact, the only Shia terrorist organization I can think of is Hezbollah (from Lebanon).
I'm not saying we shouldn't be concerned with groups that want to wipe Israel off the map -- quite the opposite -- but we should be aware of who our "allies" in the Middle East are and who benefits when we intervene. The Shia/Sunni split is just one major aspect of Middle East politics and we're not even consciously aware of that one -- how equipped do you think we are to understand the situation in its entirety?