Many reasons.
Arabica is a higher quality coffee both from a taste perspective and chemical content point of view. Arabica has 1/2 the caffeine of Robusta making it the coffee of choice for those that want the effect of enhanced focus and mental clarity without being over caffeinated and causing a jittery, nervous feeling, and heart palpitations (these are often side effects of Robusta which the general public is unaware of and believes it to be side effects of ALL coffee).
Arabica is more difficult to grow (grows only at high elevations above 1,000 meters whereas Robusta can grow near sea level).
Arabica is more susceptible to disease, in particular leaf rust disease which is a fungus that can, and often does, wipe out an entire crop. Robusta is more “Robust” and is resistant to this same disease.
Arabica has lower yields than Robusta.
All of these factors combined make the price of Arabica two times, or more, higher than Robusta.
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