The tube mostly covers north London, where the clay traditionally enabled deep lines and easy tunnelling. South London's bedrock is covered by a vast web of overground lines instead.
The tube is only one component on the much larger 1000+ station London transport system that covers the entire metro region of 24 million (ROSE - 9.3 million city proper plus Rest of South East, denser than NYC CSA, at 15 million), with a million heavy rail (non London Overground) users commuting in each day
www.clarksbury.com
One of London's problems is the high cost of staffing on the Tube thanks to the most aggressive trade unions in the country (which equates to high ticket costs also), but this in turn has led to increasingly automated service, signalling and operations, hence the larger coverage, higher frequency and better timing despite a slightly smaller ridership.