I presume GNE stopped using Pilgrim St & Blackett St due to congestion when having to later turn right from Percy St into Prudhoe Place for the Eldon Sq Bus Station approach.
Possibly also because it’s been the long term aim to pedestrianise Blackett St, so the 21 X10 etc now helpfully follow the designated Bus Loop (up John Dobson St, left onto St Mary’s Place then left onto Percy St where they can more easily turn left into Prudhoe Place for Eldon Sq Bus Station). It also means easier access for passengers alighting for Northumberland St, Civic Centre, the universities, Haymarket etc.