Without getting into the politics too much, there was a big push a number of years back to increase bus travel (and therefore the physical number of buses) in London. Partly this was to ease the considerable congestion on the tube. In central London some "core" routes have a bus every 2/3 minutes during the weekday daytime. The idea being that hopping on the bus becomes easier than hopping on the tube because there is no escalator involved.

Bus travel is cheaper than tube travel and the daily cap lower (to further incentivize use).