Inverse Square Law

It is because this is a conclusion based on observation by Newton.

Newton concluded that every mass attract each other, with the attracting force inversely proportional to the square of distance between them, in mathematics, it is F = k/r2, where k is a constant.

So, this is inverse square law.

There is no explanation why the force is inversely proportional to the square of distance between them...

With the planets to have stable orbits, they have a tangential speed, and with an attractive force provided by the star (Sun in Solar system) to account for the centripetal force, which is in the direction perpendicular to the tangential speed. So, the planets orbits around the Sun.