You answered it two times.
1) They probably don't want to.
2) Saudi Arabia doesn't do it. So USA wants them as a friend and attacking ISIS might make SA a foe. Just assumption.
But also, ISIS is Islam's war. The radicals in ISIS are united in their religion, and come from many Muslim cultures and nations. They would love to provoke the Great Satan (lol, what a joke) to rise up against them. Then they could point fingers at US oppressing the poor victims yet again. Then they could hide their soldiers and supplies and mosques in innocents' homes and mosques and create propaganda about the horrors of the US bombing innocents' homes yet again.
These propaganda and media plays are part of the scheme. Just like they were with Arafat. It's not new.