Bringing in new bus routes is an order of magnitude simpler, quicker and cheaper than introducing trams or new underground lines (the Elizabeth line took, what, two decades?) If it proves there's a demand for a better solution between those locations then fantastic, the wheels can be set in motion, but in a major C21st metropolis you can't just go "bing bang Birdseye, let's make this one puce" and throw up a new piece of major infrastructure.