There are plenty of people in Sinai who have good cause to be disaffected with the Egyptian state. It would not be the first time they have struck out at the tourist industry, which is generally seen by the locals as being for the benefit of outsiders and taking away their traditional living. No one suggested the suicide bomb in Dahab a few years ago was the work of outsiders.
It is not hard to imagine the security crackdown across Sinai has fuelled tensions in the area. This is not to condone a bombing, but violence begets violence. It is to easy (and in some cases all to convenient) to simply assume ISIS were responsible.