The digital invitations couldn't be opened in the future because nobody could understand the file format.

I’d love to have overheard his plans and just turn up wearing chrome clothes like “I heard there was a party”

Hawking waited in the room for a few hours before leaving, and no visitors arrived. He regarded the event as "experimental evidence that time travel is not possible"

Sorry Hawking the other explanation is that you just are not the cool kid and no one wanted to come to your party.