1. It solves a problem for the heaviest mobile using demographic (sexting, snooping parents, etc)
2. Its social, social apps that are engaging have the potential for exponential growth
3. Media exposure via controversy and bloggers
4. Solid user experience
5. They had technical co-founder (this is a big one) this help them get to a point of viability
6. Funding, they had access to money when it was needed
7. Luck, there is always a certain level of randomness, I guarantee that you could create an app that follows all the same steps as SnapChat and go completely unnoticed