Actually it isn't. Capitalism isn't interested in cheap energy. Capitalism is interest in cheap profits and so far there's no other energy source as profitable as oil.

Capitalism has actually fought against green energy, even after knowing the possible effects of continued reliance on fossil fuels. This site has a good summary of what Exxon and other oil companies knew and what exxon decided to do. The short gist is that back in 1970 got the first pretty definitive conclusion that CO2 emissions are a concern and they spend another 10-15 years researching the outcomes, finding out that it's an ecological catastrophe and it ends with a memo from an Exxon spokesperson more or less saying that Exxon's strategy will be to emphasize the uncertainty of the same scientific conclusion they spent almost 2 decades researching. Here's literally the one of Exxons top lobbyist confirming that they did aggressively fight the science and the early efforts to prove climate change.

The only reason we're now seeing a shift towards green energy isn't because of capitalism, but because government regulations finally understanding that oil companies (capitalism) has effectively played them like a fiddle for whatever short-term profits. It's government regulations and subsidies that are now forcing capitalism towards green energy. Capitalism has little to nothing to do with turning green energy cheaper, instead it has tried to keep us from shifting to green energy for as long as it could simply because it was more profitable to stick with fossil fuel.