The human race will never get to use time travel, and it's Hitlers fault.
Some people would ask, "If time travel were possible, why don't we see any time travelers visiting the present day?" Well, what does everyone say they would do with a time machine? Kill Hitler.
Killing Hitler isn't exactly easy, the strongest armies on earth fought to do it for years and ultimately he had to kill himself.
So when time travel IS invented, every single time a traveler goes back to kill Hitler, they fail and end up getting killed themselves.
Inevitably individual after individual will invent time travel, travel back in time to kill Hitler and fail, and humanity as a whole will never learn of time travel.