The Wikipedia page is a good start. In a nutshell:

Since the surface area of a sphere (which is 4πr2) is proportional to the square of the radius, as the emitted radiation gets farther from the source, it is spread out over an area that is increasing in proportion to the square of the distance from the source. Hence, the intensity of radiation passing through any unit area (directly facing the point source) is inversely proportional to the square of the distance from the point source.

There’s a good visualisation of that explanation which is the banner picture on the Wikipedia page.

I don’t have any better theories than the existing ones, for sure! But there is an underlying pattern that goes deeper even than that law - the principle that physical objects follow the path of least resistance links these laws and many many others.