It really isn't. Its down to ketosis. Ketosis is a chemical process where your body burns fat instead of consumed glucose/glycogen. Once you eat enough glycogen that your body doesn't need to burn fat anymore, you are chucked out of ketosis. People feel a bit bad at first (carb flu) if they eat lots of carbs and sugars as they need to detox and switch over from glycogen for fuel to body fat.
PP struggling with appetite, google ghrelin and leptin. When you eat carbs and sugars these receptors can be switched off and you are endlessly hungry. I can eat and eat and eat and never get full, or if i do, its not for long, when eating carbs.
When you cut them out, you allow the receptors to reactivate so you feel full properly. Also, when eating better quality food, you don't need as much to feel full. People get concerned that they don't go to the toilet enugh, but if everything you are consuming is good for you, will absorb more nutrients and therefore produce less waste.
This is assumiing you don't eat all the keto junk and bars etc and stick to whole foods.
I recently went carnivore and feel full and slightly hungry at meal times. I'm also doing IF, 16:8, which helps too.