Many buses, especially ones on the highway, have an extra rear axle equipped with single rather than double tires, called “bogie wheels” (as I understand it). Why?

I did some research and learned that this was to be able to handle the weight and to brake better. I don’t understand this – there are trucks of all kinds of weights including smaller than, similar to, and bigger than buses. But trucks use single or double wheels on all their rear axles, whether that gives them a total of 8 rear tires or 4 or 2. Why do so many different buses then need exactly 6?

I even read stuff in “Bus Conversion Magazine”. I think I am way beyond deserving an explanation.