Because alternate dimensions will be made when one travels through time. We're just lucky we're in the original.
Actually because we can only possible time travel through wormholes, which will only transport you back to the moment they were created. Also, they heard this was the time period you lived in.
The true reason that we don't have time travelers here is because nobody cares about the 21st century. Some time travlers will go back to 32AD to see the crucifixion of Jesus. Others will go to the 23rd century to watch the colonization of Pandora. But face it, no one in the future gives a shit about George W Bush's presidential speech!