I was brought up in a house (built in1953) with a downstairs bathroom and I regarded it as the norm. (Largely, I still do). It made sense then to minimise the distance from the kitchen (where the 'copper' provided the only means of heating water) to the bathroom, rather than having to carry all those buckets of water upstairs.
Even when there's another way of heating the water, having the bathroom downstairs means that you can have more bedrooms upstairs, which is imp0ortant for people with large families who have to live in fairly small houses.