trilogy

I noticed that a lot of authors like to write their books in trilogy. Why is it so?

I'm writing a trilogy. The first book has a prologue which tells the backstory from hundreds of years ago. I'm thinking of adding prologues in the other two books where it tells a fuller backstory. The purpose will be to hint at what the rest of the book is about, essentially filling in gaps...

The covers for my trilogy make this gorgeous triptych. Collect all three, heh.