That's because, in the old days, the horse pulling a hackney carriage could only go six miles.
From the thisismoney.co.uk
Chelmsford hackney carriage taxis can be flagged down by members of the public anywhere within the Borough.
From the en.wikipedia.org
Horse-drawn for-hire hackney carriage services began operating in both Paris and London in the early 17th century.
From the en.wikipedia.org
The first hackney-carriage licences date from 1662 and apply literally to horse-drawn carriages that operated as vehicles for hire.
From the businessweekly.co.uk
I expect it was popular when Holmes and Watson were in a hackney carriage and the driver kept bending down to peer at them.
From the guardian.co.uk
All licensed hackney carriage taxis and private hire vehicles in Chelmsford will have a large rectangular council identification sticker with its license number on the front doors.