1. You normally don't expose so many people to an untested medicine, so trials tend to be smaller than this. In this case there was no shortage of volunteers, and the larger the trial the faster it can reach a certain level of confidence in a result.
2. Normally, with vaccines, you need to wait a very long time for enough people to contract the disease and measure how effective it actually is. In this case, there are so many people infected in the general population, that reaching the required number of participants infected happened much faster than usual. In fact, when the trial was planned in the spring, it was expected that it wouldn't reach its required 164 infected participants until sometime in late summer 2021. The reason this happened so quick is that the number of infections in the general population exploded, and so the required data was available much earlier.