Because your metabolism slows down after a while to facilitate the decrease in calories? Or isn't this true?
The Minnesota starvation study indicated a reduction of 15% in RMR. That's after 6months of a continuous 1500cal diet and daily activity.
The fatter you are, the less you have to worry about metabolic slow down or muscle wasting. It usually gets tricky when trying to go below %10bf.
If you use PED's, especially anabolics and thyroid meds, then you have little to worry about, other than sticking to your semi-starvation diet.
Also, weekly cheat meals, or strategic periodic re-feeds present a toggling environment, where temporarily, the bodies hormonal signalling is normalized, restoring vital systematic function(s).