Bus lanes are too easy for the next politician to remove bus priority and allow cars back into the lane. At least with rails it's a lot more costly to remove the route. Busses also still contribute to microplastics and tire waste compared to railed trams. Trams are also easier to automate which can make employing drivers and adding trams to lines less difficult compared to buses. The rails are also more effecient as there is less friction.

I'd defintely take BRT over no transit but many cities are dense enough to justify electrified trams.