What is one reason that the light of the moon in the first or last quarter is less than half of its light on a full moon?

The main reason is the presence of shadows cast by the irregularities of the lunar surface when obliquely illuminated by the Sun.

Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.