There's lots of reasons; some of which people eluded to above.
We start with a substantial calorie deficit to trigger weight/fat loss originally, right?
And a deficit is based off of your maintenance, or the point where calories in = calories out. If we cut calories below this theoretical maintenance for a long enough period of time, we'll lose weight.
Maintenance equals the total of all the calories costs associated with a day's worth of living. These include BMR, TEF, TEA, and SPA.
BMR = Basal Metabolic Rate
TEF = Thermic Effect of Feeding
TEA = Thermic Effect of Activity
SPA = Spontaneous Physical Activity
The majority of these factors drop with weight loss, thus reducing the energy out side of the equation. If the energy out side of the equation is shrinking, and the energy in side is staying the same... as we lose weight, the original deficit is getting smaller and smaller.
BMR drops because you have less tissue to support as weight is lost.
TEF drops because you're eating less food.
SPA drops because your body moves less for biological survival reasons.
There's also an adaptive component to dieting where your body adjusts itself (primarily via hormones) in an attempt to survive times of famine.