Sorry, hanzo-senpai, didn't quite understand it the first time.
I believe earth is weak against lightning because it conducts lightning very well (earth is made up of minerals and metals wich have increased conductivity), increasing the piercing ability of lightning. Besides, lightning is atracted to earth in nature (as when we see a lightning storm), showing us that earth attracts lightning.
Water is a liquid, thus it becomes weak against a solid element like earth. In nature, the only thing capable of containing and directing water is earth, because it's imovable against water; staying rigid. Water has to "bend" when facing earth. Earth is a "enduring" element, and water a "changing" one.
*crosses his fingers that he has correctly answered the question*