Why would a duck not sit on her eggs?

Yes, and he is very protective and attentive to the mother duck. I once watched a drake (male duck) that had two females (hens), and both of them laid eggs within a day or two of each other. The drake stayed with the hens constantly, only leaving for very brief periods at a time. And when the hens would take their brief break from the nest, he followed and protected them, other times he would stay behind and guard the nests.