Why is King Lear related to nature?
I am not exactly sure what you mean by "related to nature," but what I would say is that this play uses nature as a metaphor.
The most obvious example of this comes in Act III, Scene 2. In that scene, Lear is out on the heath in the storm. He is being beaten up by the elements. This is meant to symbolize the way in which the human world is treating him -- it is beating him up and is now beyond his control.
We get a glimpse of this as well when Lear disowns Cordelia in the first scene of the play. He swears by various natural forces that she is not his daughter anymore.
So the powers of nature seem to be used as a metaphor for more human things -- for the forces that people use for their own purposes.