heartworms is preventable, why don't people give their dogs medication?

Heartworms, long, spaghetti like worms that infest the hearts of dogs and, sometimes, cats, are deadly and preventable. Dogs should take a monthly preventative like Heartgard or Tri-heart, and they will not become infested with these deadly worms. Since this medication is easily obtainable through any local vet, why do people refuse to give it to their dogs? I volunteer for a rescue group and picked up two dogs from their owner this last weekend. The owner was turning them into rescue because she could no longer care for them due to family trouble. This is what she told the rescue intake people of our organization. Well, as this may well be true, it is not the whole story. Once we got the dogs to the vet and checked out, we found that they were both heartworm positive. These dogs were strictly outside dogs (another rant I will get on if given the chance) and, as such, are at a higher risk of contracting all kinds of parasites. They should've had a monthly pill to prevent the heartworms. The treatment for heartworms is costly and difficult for the dog. Some dogs are not strong enough to survive it, even young dogs. Would you want long, icky worms living in your HEART??? Of course not, so why do people subject their companions to it when it is completely preventable? If you can not afford the medication, then you can not afford a dog and should not get one!