As noted on the site:

Instructables is a website bought to you by the partners at www.squid-labs.com. We make a lot of stuff, for business, and for pleasure. We’ve been looking for a long time for a convenient system for documenting our how-to projects, and the things we make, but it simply didn’t exist. We decided we’d have to develop it ourself, and here it is, it will evolve as we grow to meet our own rigorous demands, and those of our users. Principal in our demands is convenience – it should take less time to document a project than it did to build it.