This is because all Apple keynote events start at 9:00 AM in the morning, with the unveiling the product around 40 minutes into the presentation with an additional minute provided to cover for any delays.
The time actually used to 9:42 initially. And the first time it happened was in 2007 when Steve Jobs announced the release of the very first iPhone. Around 42 minutes into his address, Steve Jobs said
"Today Apple is going to reinvent the phone"
And then a picture was displayed that showed the very first iPhone with time set at 9:42 AM. It was in 2010 that the time was changed from 9:42 AM to 9:41 AM.