I’m not kidding. Urban areas with a high density of population are always more left-wing than rural areas. Lonely losers are the ones who become incels/groypers and shit. Workers who worked in small businesses with few colleagues were the most likely to support the literal NSDAP in the 30’s. The more people you talk to the more likely you are you support left-wing ideas. I just have to make a shit ton of friends and the revolutionary spirit will bring about communism. Wish me luck comrades. Also 
nah. unless you mean that places with more people have more total leftists than place without people… which is maybe not a revelation. though i suspect you mean that urban dwellers are more likely to be left-wing than rural dwellers. which, i reiterate: nah.
having lived in and serviced the glittering metropolises and their antipodes, what Phil Neel describes as the “Far Hinterland”, where one can find the bones of extractive industries being actively scavenged, human trafficking rings feeding into furloughed prison labor gangs, indigenous communities sharing traditional ecological knowledge and protecting natural resources… it’s pretty easy for people to become lonely losers in an anonymous, world class city and it’s pretty easy to learn the names and relation to capital of most people in a tiny rural village within a few weeks and a few conversations.
the interface matters less than the person and their will to meet and connect with others.
in many cases, for all their activism and protests, the cities are not invariably the site of history’s successful revolutions nor are they what capital formations seem to struggle to control. che’s memoirs talk extensively about the failures of the urban socialists to achieve more than symbolic acts and the successes of the rural people in recognizing the opportunities and rapdily aligning, at great personal risk, to resist batista and his murderers. by the time the revolution came down from the mountains, the tide had long turned. i personally have seen multiple wildcat teachers’ strikes start in tiny rural towns, spreading across a state only to dash upon the rocks of larger urban political machines, where the insurrectionary energy is rapidly defused into lip service for liberal reforms.
in the way that david harvey says it is easier to teach marxism to prisoners than university students, materialism and class confict is likely easier to resonate with those who are not buffered from being ground between the gears by the features of civic life. and civic life is nowhere more degraded in america than the mysterious interior that every movie for the past 50 years has been framing as a place of backwardness, incivility and evil. which, as we all know, is actually the official motto and organizing principle of staten island.
I think that different localities can effect different people, differently. Granted this is mostly moot at an individual level, because we have very little control over the social contexts we end up living in; but like humans are kind of born instinctually wanting social connection with others. It’s the inability to pursue or receive it, that leads to people learning to live without it; or hardening themselves against it.