User blog comment:Commdor/Questions for Mass Effect 3/@comment-118213-20101216230959/@comment-3268946-20110105051952

They are attacking Earth because they deemed humanity as the superior species in the galaxy. If you listen to some of Harbinger's combat dialogue when he is attacking your alien crew members, he would occasionally point out certain flaws with each species, like the lack of numbers for the drell. Clearly they still wish to create a reaper, what with the loss of Sovereign and the speculated failure with the protheans. And just the fact that they don't just blast Earth from orbit (like a few planets you can mine) shows that they don' wish to entirely eradicate the species (yet).