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Fission is a useful source of energy on earth because it relies on the natural nuclear decay of certain radioacitve isotopes such as uranium- 235. We do not have to force the uranium atoms to decay, they just do it spontaneously. The key to using fission was to learn how to moderate the rate at which the decay takes place so that enough heat is generated to make power generation a useful process but not too much which would result in an atomic explosion. Basically a nuclear reactor uses various materials, such as graphite, to control the rate of the chain reaction taking place in the reactor which in turn controls the amount of heat energy produced.
With fusion, on the other hand, you are trying to force small atoms to do something they do not naturally want to do. Because the nucleus of every atom is positive there is a natural repulsion. And according to Coulomb's Law, F = kQ1Q2/d^2, the closer the two positive charges get, the greater the repulsive force between them. So the fusion of any two atoms requires a tremendous amount of energy input to overcome the natural repulsion. Fusion can take place on the sun, and other stars, because of the high temperatures and pressures present there.
Scientists have been trying for decades to develop a fusion process here on earth, but to date have been unsuccessful in getting more energy out than was put in to start with.
In a sense, with fission you are working with nature, while with fusion, you are fighting the natural order.
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