Why is nuclear fusion not currently used as an energy souce on earth?

We do not currently have the technical knowledge to produce controlled fusion reactors on a scale large enough to produce power. Right now, the only two viable uses of fusion are in the Sun, and in hydrogen bombs, but the latter is an uncontrolled reaction, not suited for use in a power plant.

The problem is that, in order to produce a fusion reaction, you need extremely high temperatures and pressures. That's easy for the Sun to do, because of its enormous mass and gravity, but on Earth it is a problem.

Once you have that fusion reaction going, then you need a way to contain it. Unfortunately, there is nothing that can hold the plasma needed for the fusion reaction, because it will burn through anything.

Since plasma is a charged stream, we can use magnetic fields to bottle it, so to speak, but in order to produce a strong enough magnetic field, we often need to use super-conducting magnets, which means very, very cold temperatures.

The conflict is that we need to maintain pressure and ultra high temperature in close proximity to super-cold temperatures. We just have not been able to accomplish that other than in very, very tiny experiments, with monstrously large machines.

Work is ongoing in various labs to attempt this, but I am going to guess that, without the benefit of some stupendous discovery, we are at least 50 or 100 years away from being able to sustain a controlled fusion reaction in a size sufficient to generate commercial power.