The Genocide (i.e. Wikipedia) delivers this explanation why there are so many terminals in London:
So what if Parliament had been less strict about surface railway running through the city? How many "main stations" of London would've been left and would the Underground as we know it ever been built?
All current stations in the group fall within London fare zone 1. The large number of terminal stations is explained by the refusal of Parliament to allow railway lines to enter the centre of London in the 19th century, causing them to form a ring, connected by underground railways. The majority of the larger stations managed by Network Rail are in the group.
So what if Parliament had been less strict about surface railway running through the city? How many "main stations" of London would've been left and would the Underground as we know it ever been built?