Basically because "reversion to the mean" does not really apply here. Cities certainly can have very distinct and different priorities to rural areas, and in the US that is one of the biggest factors in voting (tbf most countries do have at least some urban vs rural split)
Self segregation. A city attracts a specific type of person, same with a suburb, a rural area, and all in between. Less because of political views but more because of community, schools, job, income, and culture. The result is that the largest areas are often just as unrepresentative of country as small areas.
This is not just the US. I struggle to think of any country where the cities are representative whatsoever. The closest is perhaps Stockholm in Sweden, but the other cities are not.
I mean, in Spain other than Barcelona (waaaay to the left of the nation) and to a much lesser extent Bilbao (nominally in-line with Spain at large, but with very different party makeups) for obvious reasons; the major cities voted fairly in-line with the nation. Then again, party differences in general here are smaller than the US (again with the Basque/Catalonia exceptions). Rural Castille votes at most around 65-35, not the 80-20 of the Great Plains for instance. (and interestingly, within rural Castille; it's not like the cities like Salamanca or Ávila are particularly more liberal than the villages)
I actually thought of Spain and the city that immediately came to mind is Madrid - which is an interesting case of self-segregation working in the opposite of how we usually imagine it does in cities. Madrid boomed and attracted many during the dictatorship, so now you have a population that is on average more likely to be both wealthier and have fonder memories of the past.
Jerusalem is another such case, cause for religious reasons it attracts the most orthodox.
And even when such a city is divided, the self-segregation often leads to issues that all clearly agree upon. Take France last weekend, where almost every city went from near full-LREM in 2017 to now having the poor sections go left and the stable and wealthy ones go LREM. Which is a divide, but the divide between both of those sections and the rural RN regions is far, far, larger. The communities, social networks, and economic situations people interact with are different in different places - producing different results even if them politically might end up on similar teams.
