obvious answer? because the WWE never deemed it as such.
more detailed answer? who knows. the WWE could easily do it, and honestly should, seeing as how they have two secondary titles. Triple Crown SHOULD be primary, secondary, tag, not primary, intercontinental, tag. but they haven't changed it yet, so i don't know when or if they will.
I would assume they don't recognize it because it's not an original WWF/E belt. Yeah the WHC obviously isn't but it's still a world title so it's more important. Being a Triple Crown champ doesn't really mean much these days anyway so I doubt we'll see a change made.