As was mentioned, Surefires definitely aren't the only one... But one of the most popular, and accessable for most without extra tooling (try pulling apart a Preon for Kicks...)
Like maglites, alephs (although themselves built around SF e-series measurements afaik), etc... Surefire have a standard and tolerance they work to on their lights, and separate them in to classifications (P60, E-Series...) to allow a part from one light, to be used on another etc...
They also (like maglite) never used to offer 'good' LED alternatives, and still don't support RCRs in the majority of their lights, meaning that it was a solid incandescent platform, built to a high standard, for many modders to work with to get the LED performance they were looking for.
Just my 0.02 fwiw