All US client states in the region only have the individuals at the very top - kings, some of the national bourgeoisie - who are allied with the US. The overwhelming majority of the people despise the US. Not a stable situation for an ally. The client states also have their own allegiances and enemies across the region. Those entanglements make unilateral action harder. Israel is a completely foreign entity to the region without any sort of entanglements - everyone hates them. And as you have seen over the last year, these clients have to tread very carefully with their own populations in terms of being seen openly helping the US. Israel has a population that is bloodthirsty and loves it when their military causes death and destruction in the region.