In message , Recliner
writes

Because most bus routes are relatively short, and most bus journeys
shorter still (as progress is so slow). You can go short or long
distances on the Tube -- would you prefer it if every Tube journey,
however short, cost £2.50 on Oyster, or £3.50 in cash?

That would be quite a bargain for some, given that the minimum cash fare
in Zone 1 is already £4

In reality, for a flat fare on the tube to generate the same total
income as zonal fares, there would need to be a huge rise in the cost of
short journeys and a dramatic fall in the cost of long journeys. The
latter would cause a huge shift from NR to the tube for long commutes,
as the tube would become significantly cheaper, even if a little slower.
This in turn would have disastrous implications for passengers living
closer to the centre, who would find trains already loaded to capacity
when they arrived.

This doesn't happen with buses, of course, because of the shorter routes
and slow journey times.
--
Paul Terry