Probably because they are swamped with emails of that nature. After several interviews, the employer probably makes it their own personal policy to simply not respond to those. I can almost bet that the moment they find out it's a follow to interview......DEL key.
Some employers lack to respond to some follow up emails on interviews due to accumulation of work in their offices. Some can also avoid replying if they have not made a final decision or when testing the patience or persistence of a person.