By the power of truth, I, while living, have conquered the universe.
Trilogies are the best way to sculpt a story on the kind of scale Halo is on.
Act 1 = the introduction to a crisis, the protagonist(s), antagonist(s) and setting(s) and ends with a lead-up to the second act.
Act 2 = the darker part of a story, the development of the characters where they're seemingly at their lowest point and the antagonist(s) are at their highest point. Typically ends with a large cliffhanger to set up the third, and final, act.
Act 3 = the conclusion, all the strands of the story that were set up in Act 1 and 2 are drawn together to deliver a resolution to the grand journey that has been built up.
Halo isn't the only series that's a trilogy, of course. It's just the best way to write a large plot arch, one of the first things I do as a writer is decide whether the story I'm going to write is a single book or a trilogy.