I think another major thing that has to do with that is people not finding the right dog when they visit shelters. Some people want pure bred dogs, or nice looking mutts, and dogs that have a clean record. Clean record meaning no abuse, no parasites, etc. People are weird with Pit Bulls, since they are claimed to be born "fighters" when of course they are not, so they won't buy one from a shelter if they think it won't get along with the family or other family pets if it was used for fighting. The point is that some people want to get their dogs from top quality breeders so they have a nice dog without a bad history and know it came from someone with top notch breeding dogs.
People are starting to build non-kill shelters (these shelters are HUGE too, and have parts where the separate the new dogs from the already established dogs that got over sicknesses and got rid of parasites) so the dogs don't have to die, and so that most will get a good home.
Your ignorance on this subject is truly astounding.