User blog comment:StagedDom19/Missing the Main Problem/@comment-5243655-20120713185805/@comment-24174486-20120716214744

Whether or not you can negotiate peace between the geth and the quarians is based on a "points" system determined by your previous choices: you need enough "points" in order to qualify for the peaceful resolution, in addition to the necessary prerequisites and Reputation. Destroying the heretic geth gives you these "points" whereas rewriting them does not.