Since a Time Machine has yet to be created. When one does exist we may not be able to go back to a time when it didn't exist in order to not create a paradox by messing something up were we prevent it's existence.
Maybe one of the condition is moral : Our time could be seen as very awful, and if in some way the time traveler make the old times be present again, it would easily be highly forbidden to go back in a awful moment of history.
A time traveler coming back now could very well cause a paradox. They could know that no time travelers came back to now and if they did that would cause a paradox. Or some government may steal the time machine and use it to their own purposes which would cause a bootstrap paradox.
jlc6687
