1- People buy Bitcoin because it makes them profit from it.
Bitcoin gives value, and it's natural that every human want to associate with anything that can can provided them value and it's this utility value that attracts so many increasing network of users more than anything. More importantly, the profits created by the demand benefits the community not it's developers. Many projects out there but bitcoin is the big fish because it offers what they perfectly can't offer - decentralization in the real sense of it,