Why is hydroelectric seldom considered as a viable source of clean energy?

Environmentalist block any new damn construction, even though hydroelectric is a proven source of good clean energy and very reliable. We also have LNG, which is much more environmentally friendly and it gets little attention. The reason is simple, the environmentalist are pushing for their own energies, battery power even though when a battery is depleted it contains harmful substances that have to be disposed of as hazardous material, that makes no sense to me, but there it is.