The tram supremacy doesn't lie in the inherent nature of the technology but in the way we treat it! Trams get:

• their own lane
• dedicated signals at intersections (often even priority)
• infrastructure money and thus planning effort

In short, they are (usually) treated like public transport. Busses on the other hand are too often treated like just another car that's thrown in with the rest but also has the obligations of public transport. If you treated trams like that (sharing the road, waiting behind cars) they would be even worse than busses.