When a baby kangaroo is born, they are tiny and hairless, just the size of a dime. In the pouch, they develop quickly. A preemie is similar, small, undeveloped and vulnerable. It makes sense that being close to mom's skin (which is a bit softer, and emits female smells, pheromones, etc.) would be more like where the baby should be (inside the mom.) Too bad you can't fashion an actual temporary "pouch" for moms to wear, near the nipple, just like a baby kangaroo. It would probably work even better than an incubator. Any skin to skin contact has got to be better, though, even a nice warm but possibly hairier dad. :)

The baby has been hearing mom's heartbeat for months. Even a preemie recognizes this is mom who carried me inside her.

Another factor might be confidence in holding baby. Whether it is mother instinct or just experience holding babies before, most new moms seem to be more comfortable holding babies than new dads. Even fullterm babies are more likely to relax when they are held by someone comfortable with babies.

But I suspect it is mostly baby on some instinctual level, recognizes mom.