This temperature is required to overcome the electrostatic repulsion between the positively charged protons in the hydrogen nuclei. At this temperature, the protons have enough kinetic energy to overcome the repulsion and fuse together, forming helium and releasing energy in the process.
It's important to note that this is a simplified answer. The actual temperature required can vary depending on the mass and composition of the star. For example, larger stars have higher core temperatures due to their stronger gravity. Additionally, stars with different compositions may have slightly different fusion thresholds.
