…because trams have very short boarding times.
Note the stream of cars following the rapidly moving tram which has nothing in front of it.
Trams stop only briefly in the road forcing cars to wait for only a short period so when the tram moves off there is no traffic in front of it. This makes the whole journey much faster and hugely attractive to car drivers so many will switch to trams. Buses on the other hand with longer boarding times have to pull over, during which time traffic fills up the space in front.
See this video of a Brussels tram with a stream of cars behind the fast moving tram
This is because cheap and cheerful buses ( they have to be cheap due to the higher cost of bus drivers* fuel and maintenance) have only a single front boarding door which everyone has to go through one by one, and can’t have large multiple door openings, because they are of too flimsy construction.(This also makes them noisy, vibrate and so disliked). Passengers also often have to climb stairs.
First Bus did an experiment – it over 10 minutes using cash to fully board a bus – even with everyone using the ticketing app it took 2.30 secs – not everyone will have the app and not everyone will have change ready so in reality it will take longer than this.
Trams are stretched out ( see below) and due to the different engineering and structural strength of a rail vehicle have large multiple large doors and lots of space meaning tram boarding time can be down to 40 secs meaning they can pause in the road, and not pull over like buses.
Since the world wide experience is that car drivers will switch to trams, each tram can take 130 cars of the road, meaning the tram can sweep in, and the car drivers who must use their cars get a much quicker journey in because there are less other cars – even taking into account the occasional short delay caused by tram boarding. Everyone wins.
54 cars could be replaced by one bus and driver, but a Heidelburg tram could take 3 times as many, note the large doors
(* bus drivers costs have to be shared over fewer passengers, less trips per day, and bus driving is very stressful with high turnover and training costs as a result)
There are long bendy buses that try to emulate this, but they cannot get round the bends in Bath, and are much more expensive than the cheap buses we have which still struggle to pay their way.
Also car drivers simply do not like rubber tyred vehicles, which also emit toxic particles from the tyres, brakes, and engines.