Because, depending on where you live and your riding style, a tripple is useless.
I find that the extra ring tends to create more noise/run on the front deraileur. Doubles also tend to shift smoother.
My second bike is a triple, and I would like to change it to a double. If you're concerned about hills, get a 12-26 cassette. Unless you're doing double digit grades for miles at a time, I can't imagine why you would need anything more.