Here's why:

* Mercury has a 3:2 spin-orbit resonance. This means that for every three rotations Mercury makes on its axis, it orbits the Sun twice. This unique resonance creates a complex relationship between its rotation and its orbit.

* Mercury's rotation is not tidally locked to the Sun. Unlike the Moon, which is tidally locked to Earth, Mercury's rotation is not synchronized with its orbit. This means that different parts of Mercury face the Sun throughout its orbit.

So, while Mercury does have a very slow rotation period (59 Earth days), it's not always the same side facing the Sun. This is just a consequence of its peculiar 3:2 spin-orbit resonance.