I beleive the Lab is the most popular dog because the lab is worthy. Labs are great dogs for families and singles for so many reasons that we are all aware of.
It is unfortunate that this means that there will be a lot of them in shelters. It just reflects the population distribution of the various breeds. The Labs positive attributes that make it popular, unfortunately means that people that should not own a dog of any breed will seek them out. They learn that they really aren't prepared to adjust their lifestyle to one with a live animal to care for, the dog is sent to a shelter.
I believe that what it tells us is to promote rescue over purchased pups. For those that want to compete in events requiring purebred with pedigree, then seeking excellent breeders is the path that should be encouraged for those people.