The advantage of busways is that they’re a lot cheaper to build than trains. You just need some paint on pavement to build a dedicated bus lane. All you have to really build are some nice bus stops. The big problem with trains is vertical and horizontal alignment. You can’t just lay train tracks on top of an existing road system. Cars and buses can handle much greater slopes and perform much steeper turns than trains can.

For example, you can make a busway over an existing road bridge, without any need to rebuild the bridge itself. But you can’t just slap some train tracks on an existing road bridge, as the train would be unable to make it up the slopes designed for car traffic.