A visitor to the monument describes that an inscription mentions it as having being erected in 1907.
Map of India in 1905. Drivnls.uk
At that time, the British ruled most parts of the Indian subcontinent.
Location of Nagpur in India. Google Maps
So the British marked Nagpur as ‘zero’ and computed the distance of other prominent cities from Nagpur.
The Zero Mile Stone is not, however, the exact geographic centre of the country. Post the partition of India, the location of the centre of the country shifted slightly northeastwards into what is now Madhya Pradesh. This Quora user writes that the actual location is a place called Karaundi near Sihora in Madhya Pradesh. It is deep inside a remote forest.