I am speculating here, but I imagine that while the feather is on the bird it is constantly "oiled" by the birds body. Of course you loose that when it is removed from the bird. Also, if you watch birds they "maintain" their feathers through preening and this probably spreads the essential oils around. There are some birds, such as duck and others that are probably more water friendly than turkey, but I think the below reasons are possibly why they aren't used as much.

As far as turkey feathers being used, they are 1- very abundant due to commercialization of domestic turkey and therefore cheap and easier to get your hands on, 2- are of a size that are easiy to use in manufacturing, 3- have been used successfully for arrow steering for a long, long time (they work in other words). Also, many other bird feathers are not legal to have in your possession due to federal laws dealing with protected species.

JM2C