It is because of Parseval's theorem, which expresses the energy of a signal in time-domain in terms of the average energy in its frequency components
In this case, n and k are integers, but actually these are samples i.e. discrete time intervals, so they can be considered as n*deltaT, in such case it is
deltaT is a sample time period i.e. deltaT = 1/Fs where Fs is a sampling frequency.
That is why 1/(fs*N)!
