@TomTomBinks,
It's all about appearances, he wants to look tough, but he doesn't want to screw things up completely. So that's why he's targeting the weakest, most vulnerable people.
With the exception of Iran, the countries targeted are ones with refugees fleeing Islamist persecution. These people can't retaliate, and a lot of them are victims. Naming Iran panders to the prejudices of most of his supporters who still see Iran as the bogeyman in the ME.
He doesn't want to harm arms/oil deals with the wealthy gulf states, and ignores the elephant in the room, Wahhabism. This is the hardline form of Islam practiced in Saudi Arabia, and ideologically is very similar to that espoused by IS and Al Qaida.
The real threat to security is Saudi money coming into Western mosques being used to promote that ideology. This edict does nothing to halt that, and I suspect nothing Trump does will. Profits made by his arms dealer buddies are way more important than national security. All this will do is alienate moderate Muslims and make them less likely to share intelligence.