The new tube stock is designed for it to be retrofitted, but replacing all of the signalling equipment is expensive (and likely to require some closures, both of segments of the line being upgraded and of the whole line).

The subsurface lines have interworking with normal mainline services, in some places sharing platforms with them, which makes fitting platform-edge doors very difficult if not impossible.

It's rolling in slowly. Many London lines are automated. Trains in Singapore are all automated and rarely ever have drivers. London will get to that point after they replace the existing train stock.

Driverless cars can't match the volume of trains:

http://penguindreams.org/blog/self-driving-cars-will-not-sol...

What's currently happening in London is the sub-surface lines are GoA 1 (i.e., "a train system with signals") and are moving to GoA 2 (i.e., "the train drives itself from station to station unless the driver intervenes").

The new deep-level stock (i.e., New Tube for London) is specified to be able to be retrofitted with GoA 4 equipment (i.e., "there are no staff on the train, driver, attendant, or otherwise"), but will start out as GoA 2 (Victoria, Central, Northern, and Jubilee lines are already fitted and run as such with their existing stock).