One of the factors that contributes to some questions being answered quickly (remember, not all questions are answered quickly, some are not even answered at all) is that there are many users.
Because many Stack Exchange sites, including and especially Stack Overflow itself, have many users, at any given moment it's very likely that a number of will be looking at your question in the list when it first appears there. Some of them will click on it (and their interest in it should be expected to correlate with their likelihood of being able to answer, though of course there are many other reasons someone may be interested in your question).
In other words, since there are so many users, your question is likely to be seen by someone who can answer it in a short time. This effect occurs whether or not there are many individual users who browse the site for a large portion of a given day.