Because they represent the most bilogically diverse part of the environment available here on Earth. In terms of quantitative numbers, on both plant and animal sides, nothing provides habitat like wetland environments do. These are areas that are either permanently or part-of-the-yearly soaked with water, to the point, water oozes from the soil. In some areas, shallow marshes or ponds may cover the land entirely. The water may be freshwater, salt water, or brackish water. According to evolutionary theory, life as we know it originated from the seas, from some "primordial soup" of amino acids. Would it not follow, then, that an environmental area rich in moisture would be an excellent place to foster lots of different types of life, both flora and fauna?