Navigation in central London can be bit of a nightmare, with one way systems and no entries, and the direction of traffic forced in a certain circulation route.
Those of us that are familiar with walking might find it a bit challenging.
For me there is also the safety issue, if you are an infrequent rider, traveling an unfamiliar route, with a lot of traffic, it could be a bit off-putting.
I suppose in a city of 10M, with around 2M traveling into the centre, even if it's only appropriate for less than 1%, it's still a lot of individual journeys.