This is really so simple that even a two year old can understand it. In any state there are x amount of dogs/cats in shelters, rescues and foster homes-with new ones coming in daily. There are only x amount of permanent homes available. There are always more animals looking for a home than owners looking to take them into a home. You are also competing with Craig's list where often an animal can be gotten cheaper than through a shelter or rescue. Then there is the simple fact that many people prefer puppies to adult dogs with unknown histories. If every state was a no-kill imagine how many animals will sit and wait for a new home, which would actually be hard as there would eventually be no where to put them.
While many don't like the idea of adoptable dogs being PTS, it's the sad reality of our throw away society.