Looks like we've found the culprit...

"Bell's influence led to the creation of a country inhabited by a Shi'ite majority in the south, and Sunni and Kurdish minorities in the center and north respectively. By denying the Sunni Kurds a separate, autonomous area or state, the British tried to balance the heavy predominance of Shi'ites in Iraq and keep control of the potential oilfields in their territory. The British thought that Sunnis should lead the Iraqi nation, because the Shi'ite majority was regarded as volatile to govern due to its largely tribal and nomadic base in Iraq, and hard to assimilate because of an unyielding religious bias for the 'Ali' faction of the Muslim schism."

http://en.wikipedia.org/wiki/Gertrude_Bell#Creation_of_Iraq