This is what Airbus says:

EGPWS
Caution: During night or IMC conditions, apply the procedure immediately. Do not delay reaction for diagnosis.
During daylight VMC conditions, with terrain and obstacles clearly in sight, the alert may be considered cautionary. Take positive corrective action until the alert stops or a safe trajectory is ensured.

I guess the thinking is that at night, even in VMC conditions, you may not see an unlit mountain. There was a particular route into Mexico from Houston that was caught by FOQA before someone dented an airplane. Don't remember the specifics, but the min alt. on the airway was lower than the nearby Grid MORA. ATC would often give directs over the high terrain and some crews would take the direct, disregard the GPWS and narrowly clear the terrain.