The easy answer, Spyderco gives people what they want. They are one of a handful of companies that really listen to us and take what they hear to heart.
Other considerations are the fact that they have been one of the most innovative knife companies out there, and consistently so.
The offer a wide variety of quality steels, high end handle materials, and all at a price around half of their leading competitor (with equal quality).
The designs tend to focus on ease of use and comfort over looks (although, on some models more than others). All in all, they seem to have worked very hard for their position as many people's favored knife manufacturer.