There were bicycles in the Netherlands before they were segregated, a lot of them. Then they were kicked off the roads to let "progress" (i.e. the car) go unimpeded. Now yeah, they sell that as bike-friendly, so sad.
As for the why: car and bicycle speeds are just too different. Segregation improves life for both, and especially safety for cyclists, who will always be at a physical disadvantage in an accident.