When you are a startup or an individual on your own, you don't have very much money, so the fewer parts you have to buy, the better. When you design with very few parts, everything is so clean and orderly you can understand it more deeply in your head, and that causes you to have fewer bugs. You live and sleep with every little detail of the product.
My whole life was basically trying to optimize things. You don't just save parts, but every time you save parts you save on complexity and reliability, the amount of time it takes to understand something. And how good you can build it without errors and bugs and flaws.
What does it take to design products that are both simple and powerful? Woz's interview is chock-full of lessons. Of course, it helps to be a genius. It also doesn't hurt to have a clear vision of what you want to build. Living and breathing what you're working on sure seems to be important.
These factors all play a role, but in reading how Woz designed the Apple II, I was most struck by how important constraints were to his process. Too little money. Too little time. Too little experience. All of these constraints were in play as the Apple II was developed, and all of them forced its creator to build a system that was both simple and powerful.