I can answer the question of the original poster easily.
I wanted a dog. The first thing I did was go to the local shelter. I couldn't find a dog I liked. Keep in mind, it is my first dog, I knew nothing of dogs, and they have always intimidated me. So when you go there and you see all those dogs barking/jumping at the door, and you've never had dogs before, you don't know how to deal with it. I know I couldn't, my husband even less. There were no puppies there, except a pitbul mix (and I don't like the way they look).
After that, you don't know anything about the dog. Sure, some rescues spend a lot of time evaluating the dog, but that one doesn't (I volunteered there a bit and mostly stopped because I wanted to bring everything home), and you can't know if your dog is going to turn out being a biter or not.. We are trying to have children, and we can't afford to have a dog with a doubful past - a LOT of people who want dogs do it mostly for their children.
Then, training. Lots of the dogs that end up in the shelter were from people who obviously didn't care much for the dog, and even less for training them properly. It's much harder to train an adult dog than a puppy, and that's another reason why we went for a puppy instead.
Finally, not sure about all shelters, but that one doesn't have good veterinary control - lots of animals actually die and cost thousands in bills when they leave, and when I was volunteering there one of the dogs that got adopted had a very nasty ear infection.
So, as you see, there are lots of reasons why people prefer having a puppy - easier to train, you're the one who train them and you will know if they tend to bite (and be able to correct it), and of course for childrem, they are cute.
I will probably adopt my second dog once we have more money for it, but I couldn't imagine adopting an adult dog as a first dog, for all those reasons. Also, I have a hard time getting attached to dogs I don't find good looking, and there are not that many in that category at shelters (I'm just honest, I wouldn't want to play with a dog I find ugly).
I got flamed on another forum for posting about the same thing, but it is another opinion that people often don't want to consider.