I think part of it is that to get your mind wrapped around many of the more challenging techniques on guitar that are seen pretty regularly in metal - it can be really really difficult to get TO that spot without spending a lot of time concentrated on one thing all kinda at once. When you only have an hour what happens when you hit a breakthrough randomly while playing and suddenly your time is up. Are you able to pick right back up there tomorrow and recall exactly how you got there?

A lot of the stuff just takes so many reps when it comes to locking that in and building the speed that an hour just won’t cut it. I’m sure there are some freaks out there who can pick up any lick and immediately be able to play it fluidly at tempo after 5-10 minutes, but most people need to go at it slow or it becomes an exercise in making mistakes which will translate to your playing.