If a service (I just saw Keep and thought about this: trykeep.com) has a website like that, what'll happen when they want to make it large scale? Will they move to keep.com (given it's available?) Or is it an indication that they are not confident of becoming big?
They want to build something quickly and see if people want/use it, also as early adopters don't really care about URL/Domain. If it becomes big, they will try buying keep.com