Why is simple the best?

You probably don't want to overburden yourself the first time: you might want to climb out again. Keeping things simple allows you to back up, learn from your experiences, but not be so committed that you can only continue in one direction. Simple prevents confusion. And confusion kills great ideas.

Why simplicity is the key to success?

The key to success is to focus on simplicity. Once you have a strategy and a plan on how to achieve your goal (that means having it in writing), you must stay focused through the execution phase.