It’s just a general trend that in multi-ethnic countries emerging from long periods of dictatorship or one party rule where political opposition and discourse has been ruthlessly crushed out over decades, the only common factors that at first exist to form political parties around are religion, ethnicity, and region. (Another possibility is the “strongman” party built around a popular charismatic leader and seen as an extension of him.)
Given time, adoptation of the U.S. would probably create a two party system. The problem is that Iraq doesn’t have that kind of time and it would invovle serious growing pains along the way. (probably involving a government collapse or shutdown when the legislature is unable to get a majority to pass a budget or approve vital appointees.)
Also, I’m not sure I’d want a two-party system in Iraq. The electorate might very well wind up choosing between the Islamic Front and the New Baath Party (if communists can still do pretty well in Russia, why not?)
As for PR, very few countries use it the way you seem to be describing it , with whole country being used as a single electoral district and the percentage of the national vote determining how many seats a party gets in congress. Israel is the only country I can think of that does this, which is why they have lots of tiny parties in the Knesset. (Most countries with PR also have a minimum % required to qualify for Parliament seats)
A federal system where the country is divided into districts and then PR is applies is more workable, as is a mixed system of PR and single-seat districts. Another option is multi-member districts like Japan and Taiwan used to have.
If your have time, check the link I provided.