Justin.tv (neither an online streaming live show of some guy's life, nor a streaming platform for live video) didn't exist before we were funded, fyibtw.
Why didn't the platform exist pre-Justin.tv?
1) Bandwidth prices were much higher in 2007, and have plummeted in the past few years, making the economics more attractive.
2) There were no good, stable streaming servers commercially available. Flash streaming in FMS 2 was a joke (it would fail constantly) in 2007. Fast forward a little while and you had Red5, FMS 3, and Wowza all with video streaming to a Flash client.
3) Video advertising market grew significantly starting around 2007, providing a monetization model.
Hope that explains it!
Edit: This isn't meant to be critical; I'm just curious how you would have answered back in a 2k7 interview?