Butterfly effects, unintended consequences and IF time travellers exist because time travel is possible, who is to say the travellers have the ability to actually influence what is happening in the times they visit? Perhaps they can only observe events but only negligibly affect them?
The Hitler and Holocaust scenario frequently proposed is interesting because how does anyone know that it happened and was eventually crushed because doing so thus prevented an even worse Holocaust from happening later? Endless possibilities
Perhaps humanity does not survive long enough to develop the techniques required for Time Travel.