Answer:
Because with any theory it can be changed.
Step-by-step explanation:
If new evidence is found to disclaim a theory then another theory will be made to replace the old theory. No theory can every be a law since it is a theory. Because theories can always be changed due to new evidence.
(so basically if they find new evidence about the movements of planets then that will make a new Theory)
Example: one of the early theories about how an atom looked like was By J.J Thompson. He said that an atom looked like a plum pudding. The electrons were the berries. People believed that for a long period of time since it was backed by science. Yet that was later disclaimed by new evidence which made a new theory.
(I hope this helps!)
:)