I have never seen any statistics about that at all, so I wouldn't know if that is the case. It would be interesting to see how much more people are willing to pay for an equivalent apartment with communal bathroom being the only difference. As it is, only the cheapest places have that, so there are very strongly confounding factors. Are people getting private bathrooms because that is what they want most, or because they have more money and all nice apartments have them.

And I imagine it varies a lot between cultures.