It's a matter of what we want on the horizontal axis to represent the units of frequency.

'(2 * pi / Fs)' converts units of Hz to normalized frequency in units of radians/sample. We often see this choice of units for frequency in digital signal processing and with it the DSP implementation can scale directly with the sampling rate (if we have a half-band filter for example, if we run it at twice the rate, it will still be a half-band filter). 

[![frequency axis][1]][1]


  [1]: https://i.sstatic.net/bZB8tsfU.png