I would say it is probably because Google makes it very easy to edit maps and it offers navigation. Open Street Map doesn't allow you to enter two addresses and find a route between them. (At least not as far as I can tell.) Therefore if you are living in an area where you want that ability and your local area is not yet mapped, you will map it in Google, not in Open Street Map.

Additionally if you are a business owner you are going to add your business to the map that has more users and searchers. There is little if any benefit to having your business mapped in Open Street Map, whereas having it on the Google Map could potentially bring some customers to you.