I think people

a) resent that the push for disc brakes seems to largely be driven by marketing needs
b) understand that disc brakes on road bike are only useful in very specific situations (i.e. riding in the rain a lot, running carbon rims)
c) think rim brakes basically work, are lighter, and more aerodynamic, so why bother?
d) disc brakes require marginally more maintenance and may be worse than rim brakes in some circumstances, like long descents.