The Economist took a look at why renewable energy is so often more expensive than forms of energy that cause more pollution:

• Renewable energy technology has only really been developed since the 1970s, meaning that its still in its infancy.
• There isn’t much competition in the infant industry to create competition and bring prices down.
• Renewable energy sources have their own inherent costs to them. Wind farms have to be built far from population centers on hilly areas or in the ocean. Connecting them to the grid over such vast distances is expensive.
• They also only produce energy for parts of the day, so traditional energy generating systems have to be maintained at the same time as renewables.

Read more over here.

Source: The Economist