The short answer: Because we parents can’t get ourselves to actually do it effectively.
In fact, Positive Parenting is onto one of the greatest secrets of the universe - the power of the unconscious mind to create what we give it. Our unconscious mind does its best to give us whatever we ask for even when its what we don't. This is because our unconscious mind literally cannot understand the negative.
For example, try not to think of a pink elephant. You saw it right? And then tried to delete it or replace it? But you still saw it. That means that even when we say we DON’T want something, our mind still tries to give it to us. It’s the same thing with our kids. When we tell them what NOT to do, they have a picture in their minds of the negative behavior that they then try to recreate. When we tell them what we WANT them to do we give them a positive picture and they can go try to create THAT instead.
The problem is, most of us were still raised the OTHER way (sad face) and still have lots of negative self talk and mindsets. Negative beliefs about ourselves, self judgment and the fear and anger connected to those drive our behaviors unconsciously. That means that they are literally outside of our control most of the time. So despite our best intentions, we can carry out the positive parenting instructions sometimes, but our reflexive responses often undermine our efforts.
For example, here’s a quote from a parenting advice manual on teaching positive self talk to your kids. “If your child spills her milk, you could respond by saying, ‘what a mess. You can handle it.’” That makes sense right? I mean, no use crying over spilt milk. No biggie. It’s milk.
Except, 3 years ago when my 5 year old spilled his milk, my emotional reaction was Grrr!!! And I thought, “dammit, I knew that would happen, he never listens. I just cleaned the damned floor and now it’s a mess again and I have to clean it now or it will start to stink”. A microsecond later I remembered the positive parenting approach and said, “It’s ok. I know you can clean it up really well. You do a great job with that.” I said the right thing and followed their advice but the tell was still there. He saw my face when it happened and my emotional reaction was obvious to him. It was surely better than yelling at him, but the problem was that it wasn’t congruent. By congruent I mean that my inner state was not the same as what I was saying externally. Kids pick up on that whether they are aware of it or not. Over time, I noticed him beating himself up for making mistakes even though I had been trying to respond positively. It wasn’t until I cleaned up my OWN inner talk and deeper beliefs about ME making mistakes and a whole truckload of other stuff like being respected (i.e. he never listens) and what the state of the house meant about me (my value) that I was finally able really practice the deeper spirit of this wise approach to parenting.