As per this blog (http://weblogs.asp.net/bhouse/archive/2006/07/26/NDoc-2.0-_2D00_-R.I.P.aspx), the main admin of NDoc project has decided to close it down due to lack of donations and also other reasons mentioned here.
I think it's his personal choice to do so and if other developers want this projetc to continue then they should infact take ownership of NDoc in sourceforge and help get work done faster.
I have been working on a simple file encryption utility called NeekProtect and would continue to do so as long as I wish. So it is upto the developer whether he wants to work on open source projects without any financial donations or not.