How can anyone claim not to see a touring bike, 2up, with 3 blazing headlights? 
They do not register the bike as a threat, thus they turn. A car or SUV is larger and the mind sees that as a threat that can do them harm so they pause and give right of way. But a bike is smaller and sadly is not viewed as a threat.
Sometimes the brain sees objects as getting larger but not getting closer. The usually happens to objects coming straight towards you. A baseball is the classic example. Someone not trained in sports may see the baseball as only getting larger, but not getting closer until they are beaned. The bike is often seen as growing bigger but not coming closer so once again the mind does not perceive it as a threat.
As a long time rider and motorcycle instructor, I have come to the conclusion that some drivers just do not see us. We can wear reflective clothing, have bright colored paint, use loud obnoxious exhaust, cover the bike in bright lights, crank the stereo, and even ride in a large group, but none of that matters to the one driver who just does not see us.
Condolences to the families for their loss.