Nuclear fusion has not yet been harnessed as an energy source because

Before getting an answer for it first we have to understand nuclear fusion.

Nuclear fusion is a thermo-nuclear reaction in which two  light unstable nuclei will form a heavy stable nuclei. In this process there will be some mass defect which will be converted into energy as per Einstein's mass energy equivalence theorem.

The theorem is stated as where c is the velocity of light and m is the mass converted into energy.

One take an example of fusion in sun where 4 hydrogen atoms combine to form a helium nucleus which are explained below-

                                                       -----------------------------------------------------------------------

Here is the positron.

In this process very high temperature is needed which is approximately equal to the temperature of the sun i.e

Such temperature is very difficult to initiate the reaction on the earth surface. It should be carried out in an sustainable way also .Otherwise It will cause nuclear hazards.

explanation:

newton's third law, for every action there is an equal and opposite reaction.

come closer and become far apart and become steam/gas