[1] https://en.wikipedia.org/wiki/File:Estaci%C3%B3n_SetDomingue...
In any case, over time the price of the busses goes up steeply. You need to replace a bus after 20 years, where-as trams last 50-60 years. Prague is replacing their Tatra T3 [1] trams from the 60s, not because they are worn out (they still run like new) but because they are more dangerous in accidents for the driver (they have no crumple zone and worse brakes) and because they don't have wheel chair access. Other, poorer cities have even older trams that still run fine. When they upgrade, those old machines aren't scrapped, but are sold to more eastern ex-soviet contries as working vehicles and actually put into service there. When was the last time you road in a 50 year old bus?
There is a difference, because a pure asphalt track is easy to lay, and especially easy to rework.
Train tracks, especially those running in asphalt (shared road between cars and trams), have a host of problems:
1) It's really expensive to build them - trains weigh a LOT and the rails need adequate foundations
2) It's really expensive to maintain them - you can't just go with a miller over the asphalt, because there are, obviously, the rails themselves but also delicate wiring for positioning/switch controls, signalling etc.; also, in most cases there are no alternate routes, which means you have two weeks of no service at all where a bus might just be re-routed one parallel street away.
3) It's really expensive to keep them operational: unlike with rised rails, street-level sunk rails act as sinks for everything from ordinary dirt from leaves to stones idiots place in the rail or stuff that falls into the rail from improperly secured vehicles.
4) They're friggin' dangerous hazards for bicyclists! I can't count the number of falls and crashes I had due to being forced to escape into a sunken rail.
5) Idiots with huge trucks or who are not careful when operating stuff like lifters or excavators near the overhead wiring. Happens surprisingly often that someone accidentally damages or destroys overhead wiring.
3) I know that they have to sweep the tracks, (Modern trams actually have brushes installed in the front) but when you have tram lines with a tram comming through once every two minutes at rush hour, carying thousands of people in a day it hardly seems like a large expense to have an old man in red overalls come in at night sometimes an polish and sweep out the tracks.
4) I know, this sucks. Especially when there is an illegally parked car, forcing you to lane change.
Yeah, we do that in Munich too, but it sucks real hard. Especially when the replacement buses are also in the same traffic jam created by the construction site... and all the replacement buses cost big money these days, given that there are nowhere near enough bus drivers to serve the demand.
ad 3) Yeah of course, but it's still a huge cost factor ;)
ad 4) I know someone who carries self-made stickers with penis stencils. He places these on idiot car owners' cars.