The riders are mentioned only in Revelation 9:17 because the emphasis must lie on the horses. Or better said, the emphasis must lie on what the horses do. The horses in distinction from the riders are the tools the four angels use to kill one third of the human population.
17 And this is how I saw the horses in my vision and those who rode them: they wore breastplates the color of fire and of sapphire and of sulfur, and the heads of the horses were like lions’ heads, and fire and smoke and sulfur came out of their mouths.