In cities (primarily the north east?), apartment buildings are often referred to as pre-war if they were built before the Second World War. Why is this used as a distinction?

Maybe nothing formal, but mid-century modernism (as opposed to the stuff we often called art deco and art nouveau) caught on among the intelligentsia during the war (possibly due to modernists fleeing the Nazis) and really struck the zeitgeist of the returning troops.

From a consumer perspective beyond plain aesthetics like the abandonment of adornment and historicity, there are practical issues. The avoidance of vernacular architectural styles (like steep roofs in snowy areas) and traditional methods (goodbye post and cross, hello open plans and cantilevers) make the houses somewhat rickety. The ones made by the leaders of this movement weren't so bad, but the ones that followed are just terrible. For example, while fond of flat roofs due to his origins in factory design, Walter Gropius was smart (and German) enough to know the snow where he was working meant that he needed to construct a hidden network of slopes on his NE roofs (including the one on his home) to give the snow somewhere to go. The people copying him just put on roofs right off of the prairie or pueblo, to fairly expected results. Additionally, a resource and technology boon meant that you could ignore a lot of problems, with ramifications that lesser architects wouldn't think about. For example, thick, expensive, light-blocking walls were no longer theoretically necessary once gas was cheap and easily available and the structural support was done through interior columns, but the resulting buildings were hilariously rickety and impractical once gas prices rose.

Although not as present in the Northeast, the postwar period was also the time of the "classic" suburb, I think called "second wave" by urban planning historians (or was it "third?"). These were defined by the application of mass production to home architecture, so much of what was built was cheap and not well thought out.

Lastly, we can't forget taste. Postwar architecture is very much associated with the adulthood and taste of the greatest generation and the childhood of the boomers. Much like the phrase that nobody wants to dress like his father but everybody wants to dress like his grandfather, people just don't want to live in an architectural fad from that recently in the past (remember that line in Mad Men mocking now-trendy Victorian homes as "haunted houses?"). Boomers view modernist houses as more nostalgic that livable due to those being where they grew up, Gen X and Y/millennials despise the things, and Gen Z is still in high school and thus doesn't matter on this topic. Postmodernism, especially for homes, is somehow even less practical than modernism and thus never caught on with the public, leaving prewar buildings for those who can afford that level of authenticity and contemporary revival (which is also kind of cheap in construction) for those who can't.