Why Only Four Gospels?

Some of the alternative gospels have been known for centuries, others we knew about but were unknown in their actual text, and some other texts have been discovered of which we know very little. Who decided what became part of the New Testament and what was left out? Why don't we have more gospels in the New Testament than the current four? There are some very clear reasons for this and some of these help us to understand better the uniqueness of the Christian message.