User blog comment:LDR/Possible Reaper Motive/@comment-3186471-20120224022944

I figured it was the Reapers' way of imposing perfection on the galaxy. Wiping out races they deemed unsatisfactory and taking those with potential and turning them into Reapers. In the case that species with potential rejected them, they'd simply wipe them out and wait for another, more promising species.