Renewable energy costs more because of the way the grid works. Basically all generation sources, whether coal, nuclear, wind, or solar, all feed electrons on the grid. Once those electrons are on the grid, they're indistinguishable: an electron is an electron. So to track the clean electrons that wind puts on the grid, we have to source Renewable Energy Certificates (RECs), which are issued each time wind farms put electrons on the grid. When you buy those RECs you take ownership of that clean energy.
Here is a great EPA video that explains how it works: