Number of reasons, strategic balance being one of them, they may be worried about having a nuclear Iran on one side and a nuclear Israel on the other. Could just be they want the playing field leveled.
The other reason is energy. Essentially, oils a finite resource, at some stage its going to run out. Thats not in dispute, the estimates of when it will happen are. The Saudis and co may be thinking this is going to happen sooner than we think, in most of the countries mentioned oil reserves are a state secret and not open to outside scrutiny. They could also be thinking the less they have to use, the more money they gain from selling and increasing prices as supply diminishes could offset the price of nuclear power.
http://www.peakoil.net/ for more info
I wouldnt worry about them trying to wipe you off the map, you're far to near for that to be a viable tactic, and the Saudis have always really seen you as a minor nuisance, countering persian influence being much higher on their agenda. Lebanon for example, where they are effectively taking the US-Israeli side against Shia influence.