As I understand it, getting a fusion reactor to work requires scaling up current designs. Currently fusion can only be achieved for short bursts of thermal energy, which quickly peter out. The plasma then needs to be re-created for another burst of energy. The small size of the plasma chamber means that the energy escapes too quickly to sustain fusion reactions. Therefore to achieve sustained fusion it stands to reason that we need to scale up the reactors enough that heat is retained long enough to sustain fusion reactions without petering out.

It is not merely the geometry of the plasma chamber that keeps the heat in, it is also the quantity of plasma that retains heat. Whenever a fusion reaction occurs it releases alot of radiation, which has to travel through the surrounding superheated plasma before reaching the chamber wall. In the Sun it takes millions of years for radiation to travel from the core to the surface, all the while bouncing around, trying to get out. So the more plasma there is the longer it takes, the more chance of the radiation catalyzing more fusion reactions, and the longer such a chain reaction is sustained.

The plasma chamber has to be huge: for marginal surplus energy, the ITER reactor is 7 stories tall. For a commercial reactor the volume will have to be about 5-times larger. Furthermore, the construct is not merely structural, but is a precision machine, manufactured to tolerances of +/-3 mm. For such a large machine there are very many manufacturing issues to solve still. We can build a skyscraper, for example, but the tolerances are not nearly so stringent, the materials not so difficult to fabricate, the assembly not so complex, and the project not so expensive, or requiring such expertise to accomplish. It is no wonder it hasn't been done yet.

The global physics community will have carried out over 100 years of R&D, gradually increasing the size of their machines, before the first commercial reactor is designed, around 2050.

So that is why cynics & skeptics always quote the "fusion in 50 years" timeline.