For one thing, the targets are too important to be left to just anyone. No one but the Islamic State (or possibly al-Qaeda) would dare attack the Prophet’s Mosque.
For another thing, only the Islamic State has the right kind of experienced personnel on the ground in Saudi Arabia. In the past four years, more than 3,000 young Saudi men have gone to fight with the Islamic State in Syria and Iraq. Of them, about 700 have reportedly returned home to Saudi Arabia fully trained and willing to carry out such attacks as these.
Finally, it is the Islamic State that harbours the greatest contempt for Saudi Arabia.
Since the day, two years ago, on which Abu Bakr al-Baghdadi, leader of the Islamic State, declared a caliphate in the parts of Iraq and Syria the group had conquered and occupied, he has wanted to overturn the House of Saud.