Busses are obviously more flexable wrt routes. A dedicated ROW could provide better throughput for the express routes.
Bus routes rarely get changed in a significant manner. They remain the same from decade to decade, only sometimes getting shorter or getting longer.
If you’re building dedicated rights of way for buses, then you’ve got a monster on your hands that would take more maintenance than a dedicated rail route, especially in winter.
What I don’t like is all the political interference. En-masse conversions from rail streetcar to bus were sold as getting monopolies (usually utility companies/power companies) out of mass transportation—but what happened afterwards is that whatever private operators were given the bus routes all went out of business to be taken over either by the city government or state government.