Like all commercial decisions, it’s driven by cost and profits. While the infrastructure that underpins ISDN has been significantly overhauled and upgraded since it was first launched by BT in 1986, the network has remained pretty much unchanged.
BT has invested heavily in VoIP, so it doesn’t make any sense to invest further in its legacy ISDN network. By converging all of their services to IP, BT will be able to focus their resources and energy into a technology that will allow it to meet the demands of businesses in the future.