Let there be a system of $n$ source charges and a test charge $Q$. When we say superposition applies to electrostatic force, we conclude that the interaction between a given source charge and the test charge is independent of interaction between other source charges and the test charge. Why exactly it is the case? Also why some forces follow superposition?
Force is a vector quantity: vectors add in predictable ways, so forces are capable of being considered separately or of being added together.
We say 'superposition' if force fields (vector fields of a type OTHER than force) act on some part of an object, like mass or charge or surface area. An electric field, for instance, can make your (electrically polarizable) hair stand on end, while at the same time gravity makes your (massive) hair drape downward. Only a strong electric field overcomes gravity, but it never changes gravity. We observe this, but cannot say 'why'.
We can, however, calculate the electric and gravity forces and know how much electric field is required to balance gravity. The electric and gravity fields both generate forces, and we can sum those forces though the fields are as dissimilar as apples and oranges (and cannot be summed).