One reason is market forces. Many utilities are buying power from wind farms (or buying the wind farms outright) because it's cheaper. Solar is on the rise too: in 2016, 79 percent of added renewable energy capacity was for solar, though it makes up only 34 percent of the overall renewable energy mix.
Another reason for the increasingly popularity of renewable energy is state laws. In 32 states, three territories, and the District of Columbia (but not the federal government) utilities are required to sell some electricity generated by renewable power. And federal tax credits have helped wind power-generated electricity to quadruple in the U.S. between 2001 and 2014.
|The make-up of the national energy mix. (S&P graphic; click on it to enlarge)|