The first of these two revolutions according to Harari is what is referred to as the Cognitive revolution. The theory behind this is that at some point during the lengthy development of human species there was a mysterious switch or mutation in the genetic code of one specific variant of said species that allowed them to communicate and cooperate in large groups as well as perform more specialized tasks. Essentially, the scope of things their brains could grasp became larger, reaching its apex at the conceptualization and acceptance of fiction. The ability to believe in or even to think up things that are neither real nor tangible gave them the expanded cognitive horizon with which to further their society. Secondary both in timeline and in (how Harari seems to view it) importance, is the agricultural revolution. This is simply a change from hunting and foraging to domesticating livestock and ceasing to travel for food, rather using the land to domesticate plant life for survival.
His explanation for the dominance of sapiens is quite simply the ability to imagine things and unite under them. This gives sapiens the ability to operate in larger groups with more efficiency and thus accomplish more difficult tasks. His arguments make sense in principle, however we do not often see random genetic mutations causing a gain in information or an overall increase in functionality. This perspective also ignores the possibility that Neanderthals were peaceful with sapiens or shared a certain intelligence or competence. To use and example, the Aztecs were a very intelligent and advanced society yet they vanished from the face of the earth. Of course, they warred with the Spanish and fought disease but their civilization disappeared seemingly without trace and is still somewhat inconclusive for historians today. How much more difficult then would it be to say with absolute certainty that one of these two groups was less intelligent or died out as a result of the other? And as for imaginative thought, once again who is to know for sure what they were capable of? I could go on but for sake of excessive word count I shall leave you with those thoughts. As to why the sapiens became dominant? The obvious answers include our superior physical abilities and intelligence to use those for our gain.