Persia and India

Why did the Persian Empire under the Achaemenids (especially at its zenith) never invade India beyond the Indus Valley?
 
The Ganges is quite far from the Achaemenid centre of power in Mesopotamia. Also, it's quite hard to hold onto India, and to hold onto the Indus Valley (Hindush satrapy) alone, the Persians needed to grant so many concessions that the satrapy was virtually independent.
 
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