I believe the reason is because the moon gets a tiny bit closer to the earth ever year.
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When the moon grows smaller, it is said to be waning. This is the period between the full moon and the new moon, when the visible portion of the moon decreases.
When the moon appears to be shrinking, it is said to be waning. This phase occurs as the amount of illuminated surface visible from Earth is decreasing. It gradually transitions from a full moon to a new moon.
Waxing Moon.
waning
The last words spoken on the moon were "Ignition," said by astronaut Eugene Cernan just before liftoff of Apollo 17 in December 1972.