Because rainwater is the best water you can use.. Its not hard at all, typically needs little pH adjustment depending on your feed, and brings some micros to the party too.. My rainwater is typically pH5.7 and ~17ppm.. When soil has been properly limed it works out perfectly.. Oh, and its free, just set up a collector under your spout, and keep the sun from pummeling it as best you can to keep algae and whatnot from coming around..