The advantage buses have over tram and light rail is their ability to be in mixed traffic and have a flexible rout layout. If you go for a dedicated rout than buses loose out against trams and light rail, on speed (yes rail is faster), energy energy efficiency, comfort (smooth ride with no swaying), and operating cost (fewer maintenance cycles and longer lifespan of the equipment)
Buses are best suited for the lower capacity transport routs in places that have lower density of people living there, and as gap fillers for temporary peak transport needs.
Oh by the way i contradicted you on the speed issue because if your compare a 3 compartment segment bus (with 2 hinges) with a comparable capacity light rail, the light rail can go over twice as fast.
Now for the ideological reasoning, capitalists don't like light rail, because the labour inputs have a higher value add. Meaning you need to train people more, but they also add more economical value. Capitalists that mostly are short term minded don't like that rail has high initial capital cost and represents long term investments. Socialism does not have it's investment strategies limited to short-termism, but rather can select strategies for investment based on material conditions, and obviously prefers high value-add labour, because that is more economical.
For Areas with sufficient capacity needs, to justify rail construction, that is the preferred mode of transport, and obviously buses would be used for the lower density Areas where rail construction is not justified. There also are "trackless trams" (pic) that combine some of the features of buses and trams which might be worth considering as well.