Less Advanced Societies Kept Unaware Of Time Travel

In order to limit the ability of time traveling criminals to prey upon those who had heard of them, or to specifically target those who were aware of them, many massive life support systems began limiting perceptions of time traveling criminals and their depredations; they also did so in order to simply make those societies more comfortable until help could eventually arrive.

By maintaining Earth as apparently low tech, or as less advanced in appearance, time traveling criminals have been able to argue that they should be rescued from Earth, while limiting investigations of it, the original planet Terra, in the original timeline of the universe.