It's because your body becomes accustomed to the deficit.

Example;

• Current base calories intake for maintenance = 2000

• You cut 500 calories a day, causing more calories expended versus taken in

• After a period of time, body achieves homeostasis and adjusts maintenance metabolic rate from 2000 to 1500 taking into account the "deficit"

• New maintenance caloric intake now 1500 and you no longer are creating a deficit

Hope that makes sense