No idea. Probably cause it’s a bit gate-keepy in the way I say “any tuner worth their salt” as if it’s the only way to achieve good results.
I haven’t met a tuner that uses anything other than forks. Maybe that’s because all the pianos I’ve worked with have been in good condition, so haven’t needed drastic measures applied. As I haven’t met a tuner that uses anything else, I can’t say if they are better/faster/whatever. I just assumed it’s the industry standard, like how orchestras tune by ear