Peace-keeping = occupation in the middle east. That means the region's largest Sunni power player occupying a zone where the Shia hold influence via proxies (Iran funding Hamas). That would be Iran's arch-nemesis militarily encroaching on its turf in the eyes of Tehran. Iran would also oppose that kind of precedent more generally of Saudi Arabia becoming a regional peace-keeping force. What would stop the Saudis from threatening Iran's Shia-dominant proxy governments in Syria or Iraq via "peace-keeping" for example? Iran doesn't want that Pandora's Box opening up and would fight it tooth and nail--including local Gazan insurgencies against a Saudi peacekeeper force.

The US was functionally a peace-keeping and nation-building force in the wake of Saddam's removal in Iraq and look how that went. Civil War, power vacuums, competing interests, an insurgency against the peacekeeping force *and* the new government. See what I mean? That kind of thing doesn't go cleanly out there. The same would happen with the Saudis acting in Gaza the same way the coalition acted in Iraq.