Sunnis are anywhere from 20-30% of Iraq. The government is set up based upon ethnosectarian quotas so Sunnis are represented throughout the government. They get the speaker of parliament, one vice president, one deputy premier and various ministries. This has been true since the 1st post invasion government formed in 2005. The Shia will never except IS as a legitimate entity nor include it in the government since a main part of the Islamic State's ideology is the destruction of Shiites which they see as apostates.