What if those time travelers were smart enough not to go to the party because they might cause a rift in time and space because there was a change in the past. But considering the recent time travel paradox, if the time travelers were to go to the party then they will cause a change in the future which is going to change the timeline where the time machine was invented earlier or later which means that they didn't really went to the party resetting everything.
Funny how Stephen Hawking never said who the invitations were sent out to - he refutes time travel but knows Time Travelers enough to be able to send them invitations? That's got to be a paradox.
Of course it might just be no-one wanted to go to Hawking's' party.