How would any time travelers have known about his party? Did he advertise it? Or did he assume he would invent the time machine? Maybe time travelers just went to a much cooler party where there were drugs and people other than Stephen Hawking.

It depends on the type of time travel rules. One theory suggest going back in time creates a branch in the time-space continuum, one where the time travel happened, one where it did not.

With this branching time-space rule, there's one timeline, where no time travellers appeared, and one where they did.

A party with no bitches, just some nerd who doubts your existence and would spend time asking you stupid questions? No wonder no one came...

Plus it would likely be taboo to visit the past because of how easy it'd be to spread future germs and get everyone killed.

What if time travel requires an anchor so you can't go backwards before the time it was invented?

Well, given what we have learned about Hawking, it's not that shocking. Also, why would they reveal themselves like that.