Maybe to not disturb your SO with flushing/shower etc? If I will ever building a house toilet & bathroom will have to be as far from sleeping parts as possible.
Quality of deep sleep is far more important than walking 5m extra from time to time
The house in Germany we finally bought features an almost perfect solution: the master bedroom is alone on the top floor and the only other room on this floor, separated by a 2sqm corridor, is a large bathroom. So you actually have 2 doors available to separate sounds from the bathroom from the bedroom and still have the privacy of an ensuite-bathroom only for the owners (as none of our frequent guests would "by accident" get to our floor.
Quality of deep sleep is far more important than walking 5m extra from time to time