Trams scale to higher capacities than buses, given the same vehicle width a dedicated transit lane takes up less space for trams (and can be built with green track) than a comparative busway, a highly used dedicated busway isn't that cheap to build either because buses with their high axle loads do pound the pavement just like trucks/HGVs, all the more so in the case of a guided-bus way in case you try to reduce the width requirements and built some sort of "rubber-tyred tram system", steel wheels on steel rails are more energy efficient, …