There are many reasons for this. Some are cowards and don't want to talk until they want to break up and be "free", some don't want the drama, some find someone else, some just get tired of the person that they are with and so on. The confrontation of a break up is never easy for the one that thinks that everything is going so great. It is never out of the blue for the one doing the dumping, it was carefully thought of. Then there are some that love to hurt people intentionally.
Because sometimes it's just easier to let go.
Why do people not say how unhappy they are and then dump you out of the blue?
Instead of just communicating in the first place?