Spines are mobile in three axes: extension/flexion (front to back), rotation, and lateral flexion. (side to side) For a lot of people, deadlifts can involve some extension and flexion and remain safe so long as there's no rotation or lateral flexion. It's ideal if the flexion/extension stays consistent through the motion (even if it's not completely neutral)

That's why it's possible to safely deadlift 500lb then go home and throw out your back putting a 50lb box on a shelf at your side. It's possible to train yourself to lift from awkward spinal positions safely, but it requires starting at a lighter weight and progressing carefully. Gladiator deadlifts are a neat way of introducing flexion + rotation.

I think it's the way the weight is distributed. It's easier to keep a barbell closer to your center of gravity, so picking up a more awkward load causes you to activate your back more.