Couple of reasons:
1) Too much dependency on investor capital. As an investor, anyone would want returns in say 3 years, that too minimum 60% upwards on a realistic scale. This puts unnecessary pressure on the system.
2) Response in adapting to changing market demands. Slower the response, faster the start up will die.
3) Problems which are sometimes out of scope of founders i.e. government policy changes which impacts the business directly.
4) Lack of strong ethics. This is one aspect many startups are currently lacking. Most of the old companies have a strong ethical foundation.