Because plans developed (and there have been several successful plans) to use free, or very inexpensive fuel sources (water, solar, etc) have been squelched by the big oil companies. They pay, whoever came up with the idea, off. Look at the extreme wealth of a few in the oil producing countries, it's nothing for them or, say, BP, to pay millions to cover up an idea that would stop the flow of oil & money!! If you came up with a great idea, that's easily workable, to use saltwater instead of gas by making inexpensive changes to a car's engine and someone swooped in with a 50 million dollar check to buy your idea & shut you up, wouldn't you sell out? If not, I'm sure they have much more devious ways to keep it under wraps.
It was the first fue ever used nationally, from the deep coal mines in America. We still use it because no one has yet come up with a good way to use wind, solar, or other. It's not yet practical, too expensive, to use Green ways nationwide.
We get so much of our energy form fossil fuels because they are easily combustible and are inexpensive. Another reason is that these fuels are widely distributed and can release large amounts of energy. Read more at http://lifeofearth.org/fossil-fuels.
Nothing else even comes close to being viable financially. It is 20-30 year payback to install solar and try to come out on it vs buying electricity generated by coal or nat gas. Wind is even worse it might break even in 30 years about when the windmill is worn out.
they are easy to use and are cheap. they are also efficient, but less efficient than nuclear power (which creates toxic waste) and hydroelectricity (I missed the hydro electricity pros and cons lesson in my sixth grade science class)