Because of ecosystem and water footprint impacts associated with dams, in conjunction with the operational requirements of power generation. Dams obviously convert a river into a lake and impede/alter natural migratory patterns. Less obviously, hydroelectric power generation dictates a pattern of water release that is at odds with other demands on water usage, for human and natural usage purposes. (You can imagine that downstream environments might benefit from water flow which is more steady, or which follows natural rhythms; this is in conflict to some degree with grid demand for power.)
