Here's why:

* Newton's Law of Universal Gravitation: The force of gravity between two objects is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.

* Sun's Mass: The Sun is incredibly massive, making its gravitational pull very strong.

* Distance: Mercury, being the closest planet, experiences the strongest gravitational pull from the Sun due to the inverse square law.

While the Sun's gravity extends throughout the solar system, it diminishes rapidly with distance.