The answer to this question is very simple: because we don't knw how to do it yet.
All the existing nuclear energy plants work based on the fission of heavy elements (like Uranium) and not on the fusion of hidrogen (like the H-bomb).
There are some research taking place in Europe (the ITR project, in southern France) in order to build a prototype that produces energy based on fusion.
The main problem faced by the scientists seems to be the difficulty in maintaining a plasma gas (required for the fusion) in a container that resists the enormous temperatures involved in the process (we are talking about millions of Kelvins).
A fusion reactor would replicate, in a smaller scale, the processes that take place inside our Sun. Once operational, this technology would solve our energy problem in the foreseable future.
:-)