What is the main problem with fusion?

Could you elaborate on the primary hurdle that scientists and researchers face when attempting to harness the immense power of fusion as a viable energy source? Is it the extreme temperatures and pressures required to initiate the reaction, the complexity of maintaining a stable fusion plasma, or perhaps the daunting task of developing economically feasible fusion reactors? Understanding the main challenge in this field is crucial for advancing towards a clean and sustainable energy future.