1. Since the quilt is used in combination with a sleeping pad, there's no "wasted" weight from having sleeping bag insulation compressed underneath your body. Less compression in your insulation means more warm air can collect, so quilts basically eliminate the compressed stuff and replace it with a sleeping pad. A sleeping bag has both, but weighs more.

Many people also find quilts more comfortable since they restrict movement a bit less. I personally find them a bit finnicky to keep tightly sealed for warmth, but I am very skinny.