Quick and cheap construction at the expense of everything else. Definitely inferior liveability, but back then, the standard was lower, so having indoor plumbing already made them "luxurious" compared to turn-of-century ganghouses.
And especially relative to co-habitation, where multiple families lived in the same apartment, each having a single room for themselves.
They are very 20th century in a sense thry represent a failed promise of modernity.