Because locks are heavy, and the place I park most often is the place I (usually) want my heaviest, most secure lock. I always carry a lesser (lighter) one for short/unplanned stops, but given that I have to park in essentially the same place every day I ride to work, it makes much more sense to park the heavy lock. Of course, my bike rack is inside an underground, attended garage, so the only real risk is the landlord.