For several reasons in my case. Primarily, the location is a new partition wall with no existing fixtures, so I'd have to shell out for the bulbs and fixtures (and man, are decent looking light fixtures expensive!).
Also, I wanted something that would fit well into the spot visually and not protrude into the hallway much. It was just difficult to find a fixture with the shape I was looking for
I also had most of the materials already available - Plywood, acrylic and the frosted glass spray were left over from previous projects, so that made it quite a bit cheaper, too. For the kitchen, I plan on building because again, I'll need new fixtures to replace the pretty ugly ones in there right now. I've still got enough acrylic, which should make the total cost for three similar fixtures for the kitchen about $80 or so, everything included.