Black cabs pay a lot of money to councils for their licences, more than private hire.
Private hire charge by the mile, usually, whilst the black cab fares costs can depend on journey time, so if it is stuck in traffic it costs the passenger a lot more.
"Hackney Carriage vehicles are permitted to use the bus lanes in order to speed up transportation links, this type of taxi is easily identifiable by the bubble displayed on the roof of the car. One of the functions of a hackney carriage is for it to be flagged down from the street. To do this may require it to enter a bus lane. Private Hire vehicles do not display the bubble on the roof so therefore are not easily identifiable. This type of taxi is pre-booked and cannot respond to passengers flagging it down in the street. Consequently Private Hire vehicles are not permitted use of the bus lanes. The distinction is made to cater for the different type of taxi and to enable Police enforcement of the bus lane restrictions to easily take place."