When you come out of a lift at the centre of the building (for the reasons of construction that DA01 mentions), then you typically have options of going in 2 or 3 directions in order to get to your room.
A communal convenience should therefore naturally be placed where it's equally convenient for a person, irrespective of the direction of their room. To position a convenience somewhere along one route, makes it less convenient for people on other routes, who continually have to deviate.
It aids discovery - subconscious discovery each time you pass, lets you notice it even when you don't need it at the time, but you recall where it is when you do. It also naturally follows that you position them in the same layout irrespective of the floor number so that guests or tenants always know where to find it on any floor they happen to be on.
Additionally, few people want a communal convenience next to their own room, so centralizing such services including storage and cleaning rooms close to the elevators ensures that no one guest or tenant has to suffer the frequented footfall and use (or abuse) of conveniences 'in their own backyard' so to speak.