Cause the rod companies tell the comsumer that they really do want to buy "fast" action rods. I believe in the power of suggestion. People will buy the latest and greatest because they think that the manufactures know best. Shoot, if Sage designed it, it must be the latest technology. I gotta have it, it will make me a better fisherman.
Personally, as I have progressed in my own fly fishing journey, I have migrated more toward "slower" action rods. I can feel the rod load better and lay down softer presentations. I went from a fast action Sage to a lower modulus rod, to fiberglass, and finally Bamboo. If you let it, a slower action rod will just about cast itself. I do feel a connection to Bamboo and fiberglass that I don't get with graphite. Ya ya, I know next thing, I'll be wearing flowers in my hair and talking to the trees.
A fast action rod does seem to be better at punching thru a stiff breeze, but that is about the only advantage I can see. A "fast" action may also be a little more forgiving of bad casting habits.
I also tend to steer customers away from the "fast" action high modulus blanks. To me, high modulus equals more prone to breaking if the rod is nicked or scratched. They are strong, but don't seem to have the tensile strength of lower modulus graphite, fiberglass or Bamboo.
Just remember all my opinions are based on me being a slower action junkie. No scientific facts to back any of it up.
Doesn't matter if it is a slow or fast action, people need to learn not to fight the rod.
Sorry, let me step off my soap box now
Pete