What makes you think that the THX certification for wire and cable products was conceived with the custom installer in mind? Any custom installer worth their salt knows how to identify cables and uses techniques other than the THX color coding. In the consumer world, which is where this was REALLY aimed, the THX schema for color coding has been, for the most part, superceded by widespread application of the relevant CEA standard.

Were you there "...in the beginning..." to witness what you are speculating? Have you read the specs?

While THX testing for certification is a precise process, it is no more daunting than the many other certifications a modern audio/video product must have. The process, itself, is not the problem, no more than it is when submitting a product to Dolby, DTS, UL/CSA/SEMKO (etc.), FCC, HDMI, etc.