Using nuclear fission and fusion for generating electricity requires very different technologies and engineering. With fission, the splitting of heavy nuclei (uranium, plutonium) happens quite easily – and most reactions accelerate (i.e.. produce more neutrons for splitting more atoms per reaction). So harnessing fission (in conventional nuclear power stations) needs engineering to moderate the reactions and safety systems to cope with accident scenarios.
Fusion is quite different. Forcing light hydrogen-like nuclei together does not happen at room temperature – indeed we need to exceed the temperature at the centre of the sun to make it happen (100 million degrees Celsius). For us, the challenge is creating a very hot gas of fuels, controlling and confining it and getting the fusion reactions started. This is why fusion is still in the research and development phase – and fission is already making electricity.