Because bus lanes are usually poorly implemented. Usually they just take out a car lane, call it a bus lane, and wash their hands of it. Car congestion is now worse, and buses still have to wait at the same lights.

My city did a bus lane right: they took a 3 lane road (2 lanes each way plus dedicated left-turning only lanes), eliminated the turn lane and turned that into a dedicated bus lane with dedicated switching. The bus lane switches don't correspond to regular traffic lights, so drivers don't pull into the bus lane to use it (think weird slashes and circles) and the buses don't have to wait at stoplights next to regular cars. Buses fly by the traffic, and cars didn't lose a lane. They did extensive traffic surveys and found that only a small portion of traffic was turning left across oncoming traffic and those cars spent a long time waiting anyway. So now cars have to drive a few more blocks to turn but it's almost the same travel time since they were waiting so long before anyway.

They also added bike lanes with raised concrete medians between them and the cars, which double as platforms for bus stops.

It was a huge win for everyone and works really well.