This is a bit above eli5 but a simplfied version is because of relativity the faster you go the slower time happens. At c time stands still, if you were to go faster than c time would happen backwards. And with our current understanding of physics we dont have the slightes clue what time going backwards would look like. Also even though most people know c as the speed of light its actually not, light has diffrent speeds depending on the medium it travels through, the fastest is through a vacum which is c or the speed of causality.