"50 top startups" is an unrepresentative sample of the ecosystem. It's not clear what criteria were used to place startups on this list, but almost all had entered the Valley hype cycle, indicating heavy-duty funding and/or a paid PR push. If their funding runway was long enough, and they were able to get follow-on rounds (or be acquired/achieve profitability), it shouldn't come as a surprise that just 12% have hit the deadpool.