Honestly, I think its because guitar tones for these genres have really not evolved in 50 years. When people talk about a great tone in thise genres, I would hear "oh man, that tone that (Clapton/SRV/Knopfler/Santana, etc) got in that (1970-80) record was the best tone I've ever heard". I can plug into an ancient amp and get a great blues or country tone. Can I get a great metal tone from one of those amps? No.
Metal has always been about innovation and evolution. The tones you heard in metal less than 10 years ago are already outdated. Controlling a large amount of gain while minimising noise, maintaining clarity, harmonics, definition and grain is a challenge for which you need a sophisticated rig like the Axe FX.
While the ease of use makes it an attractive tool for a blues or country guitarist, you REALLY dont need an Axe FX to play those genres. And don't mistake me for some noob - I speak without any bias. I'm over 35, and I have been playing guitar on stage for over 20 years in blues, pop, rock, reggae, covers and metal bands.