Why is the decision of majority better than the decision of minority?

Because the majority represents a significantly larger portion of the population than any given minority. We view things in a utilitarian way - the most good for the most people. That doesn't mean that we always let the majority do what they want, after all tyranny of the majority does happen, but we look to the majority because they represent the most constituents in a given population. Simple as that. The majority doesn't necessarily know best, or even really understand what it wants, but their opinions end up mattering most.