Because they are a popular breed.
More on this phenomenon: http://www.blackpearldogs.com/Also, big black dogs are statistically harder to adopt out. There are many theories as to why this occurs. So, these dogs come in and do not get adopted out.
Yes that's true! It is partly because when they get them as puppies they tend to forget they're small and will get bigger not only that they are highly active and are puppies forever until they're 5 years old. They need to be trained daily & need to be exercisedand I think when they get them they realize after what high energy they have they don't want them anymore which is sad. My dog is not a Lab but she is high energy but giving her up just isn't an option