[pandaman]

The article actually explains it: with 2 staircases requirement the cheapest[+] option is to have the internal shared corridor and thus you get just one external wall for an apartment in general (unless it's in a corner where it gets two). If you arrange a bunch of bedrooms along the wall you end up with an internal corridor, wasting space, and making your whole apartment looking like a sleeper train car.

Houses, on the other hand, have 4 external walls at the very least. Not to mention most 4+ bedroom houses are 2+ stories as well.

[+] It might be also required to have staircases at decent distance, as a second staircase going through the same stack of apartments is not of much use as it's likely to be blocked by fire at the same time as the other one.