Robusta coffee tends to taste more bitter because it contains higher levels of caffeine and chlorogenic acids than Arabica. These compounds naturally create a stronger, sharper, and more intense flavor profile.
In addition to bitterness, Robusta often has a heavier body and more earthy or nutty notes. When roasted and brewed well, that bitterness can feel bold and purposeful rather than unpleasant, especially in espresso blends and milk-based drinks.