Again, time. The faster vehicle occupies more road but for a shorter time. That fast car down the road and that closer/slower bicycle both occupy relatively similar amounts of roadway in comparison to multi-person vehicles like busses. The fastest way to move people down a given lane is to cram them into trucks army style and then drive slow enough that each truck leaves minimal time between each. A lane can then handle a 50-person truck every 5 seconds, or 10 people per second. That's far more than can be pushed through on individual bikes riding in pairs.