Here's why:

* Nuclear Fusion: The core is where nuclear fusion takes place, converting hydrogen into helium. This process generates immense heat and pressure, squeezing the matter together incredibly tightly.

* Extreme Conditions: The core's density is estimated to be about 150 times that of water on Earth, with temperatures reaching millions of degrees Celsius.

* Gravitational Pull: The Sun's massive gravity pulls all its matter towards the center, further increasing the density of the core.

While the other layers of the Sun also have significant density, they are significantly lower than the core's.