Please excuse my ignorance - the other morning I was watching the aircraft lined up on taxiway bravo for a 21 departure, and I couldn't help notice that the next aircraft to take to the runway seemed to take forever to lineup. I looked into the distance and couldn't spot any arriving a/c. Why isn't the next cab on the rank moving to lineup as soon as the one cleared for takeoff is rolling? It must be procedural and I understand the wake turbulance but these where all 737's.
