They are also harder to figure out. It's pretty easy to figure out a streetcar / light rail line: the station is the big thing with the platform, it travels along the tracks.
With a bus you need to know where the stops are; with a train line, you can just walk until you find the tracks, then walk on the road nearby until you find the station.
If you see a station near your point of origin and a station at your destination, you already know a lot of useful information about the transit system without needing to look anything up.
You see trains every so often so you subconsciously absorb whether they run weekends, how late, etc.
With buses there are more variables to consider (is that bus I saw out of service, does it normally run on this route, etc).
In addition to usage based service changes, a bus route can be diverted to random alternate streets for events. The train route can't, so they divert the special events away from the tracks.