One of the single most important factors in ISIS's resurgence is the conflict between Iraq's largest two Arab religious groups: Shias and Sunnis. ISIS fighters themselves are Sunnis, and the tension between the two groups is a powerful recruiting tool for ISIS.
In the most basic theological terms, the Sunni-Shia split in Islam originated with a controversy over who would take power after the Prophet Mohammed's death. Today, of course, Iraq's sectarian problems aren't about relitigating seventh-century disputes; they're about modern political power and grievances. But those do tend to fall along Sunni-Shia lines.