From the early failures, we've learned that we need end to end control in order to make products successful. So I guess the simple answer is experience.
> Why are full stack startups emerging now as opposed to a decade earlier or later?
Fifteen years ago it was hard enough to create software that for most people doing it, it was their main if not only area of expertise. Whereas now it's easy enough that even hobbyists can create high quality software while still specializing in business, marketing, manufacturing, operations, etc.
[1] http://a16z.com/2015/01/22/the-full-stack-startup/