These reasons are (casually) mentioned in the post:

* "the deeper your reservoir the more efficient the water gets pumped. As the water gets consumed, the pump gives less output"

* "If the reservoir runs dry the motor won’t catch fire. That apparently is a thing water pumps."

* "Since the water is only going through a simple tube and not an expensive motor, you can pump a nutrient solution. If you want to pamper your plants, we don’t."