Why do some Partners fail and others not?

It is not because I have the answer, this is just a thought.

Several thousand books have been written on Partner and Channel sales methods and techniques.
With all of this available information, why then are so many partners struggling to meet their quotas?
Why are so many companies struggling to sell enough products and services to make a profit?

A cynic or pessimist might say that the products and/or services of one the one partner that doesn't make the quota are just "low quality", that they are unneeded, or that they are impossible to sell what they have.

Personally, I don’t buy any of those suggestions or excuses.

There are thousands of partners (companies) that go out of business despite having outstanding products and or services.

Why do these Partners fail......is there approach, method or technique wrong?

Regards Mark