I think the answer to this question boils down to cost. Although the sources of alternative or renewable energy are generally free, the equipment required to convert the energy to a useful form (such as electricity) can be expensive. This means that it might take a very long time before the money you save by not using electricity from the plug is greater than the cost of buying the renewable equipment. Often, even though we know that using power from fossil fuels causes problems to our environment, we are not prepared to pay the extra cost need to generate the power from alternative sources.
However, things like solar panels are getting cheaper all the time, and in some places it now makes more sense, based on money alone, to use energy from the sun than to buy electricity from a fossil fuel power station.