It's not just one of the most prolific, as far as pure breeds are concerned it is the most prolific by a big margin, it has been the most popular breed registered with the American Kennel Club for the past 24 years and Labrador retriever puppies registered outnumber the next most popular breed, currently golden retrievers, by about 2:1.
The reasons for its popularity may not have an explanation beyond the whims of fashion, the American public is quite fickle about their favourite dog breeds, changing their minds every decade or so, the Labrador has had an unusually long run at the top spot thus far.
Breed guides certainly make it sound attractive, it's often described as being versatile, easy to train, good with children and other pets, usually having robust health etc. However there are plenty of other attractive breeds, so this is not a satisfactory explanation. It may just be a matter of taste, which does not lend itself to any deeper analysis.
http://bigstory.ap.org/article/labrador-retrievers-set-popularity-ranking-record