As I always understood it, the 747 suffered from being underpowered in its youth, and its sheer size with the market the way it was at the time meant that the DC-10 looked like the more sensible choice for airlines for quite a few years. However the 747 slowly worked it's kinks out, and the DC-10 suffered some high-profile negative publicity towards the end of the '70s. The result was that the 747 became something of a sleeper success.
I also think the A300 deserves an honourable mention for being the first high-capacity twinjet. While it didn't corner the market on its own, it became the rough blueprint for later workhorses like the 767 and 777 and the birth of ETOPS.