The short answer would be because it is hard to make a serious error as with other forms of dog training.
The long answer would be because you are working with your dog and learning to communicate to them effectively by developing an understanding of canine learning and how to show your dog what you want in an easy, stress free manner.
This is effective because your dog is not under a great amount of stress and is not "pressured" into shutting down, avoiding you, or becoming afraid.
It's also effective because you are learning how to communicate with your dog and that is the key to making any training method successful.
Positive dog training is fun, exciting, challenging, and the pay off for those who stick with it is much greater than the people I call "McDonalds" owners.
Or people who want fast food results when training their dog.
Fast food results are not possible with dog training because dogs are animals and it takes time to teach them.
These methods are easier to use for most people and as I said in the first paragraph positive training is hard to mess up.
The results you will get from positive dog training will far outweigh the results coming from any other method.
Once you understand how to show your dog something you want them to do the rest is easy as pie.
Obviously the more complicated the behavior the harder it will be to communicate what you want to your dog and this is where positive dog training works better than any other method out there.