There's nothing really technically special about Uber so I doubt that investors were lured by the strength of the proposal or anything like that. The founder of StumbleUpon was a co-founder, so it was most likely through his connections + track record, but I don't actually know the details (someone else here might). I would guess that it's just an accident of luck that Uber became Uber -- right time, right place, and right connections. That's what it takes to get a business off the ground, especially a business that raised $1.5 million in funding before it was released (per Wikipedia's timeline).

Kalanick's previous company Red Swoosh sold for $19m. They were probably backing him as much as anything.