Thanks, Elizabeth - it's been fun for me too!
I think that historically, evangelicals have championed overseas missions in a way that has not been true with domestic urban ministry. We've romanticized the heroic overseas missionary, going to the far corners of the world. At the same time we've had something of an anti-city bias. Evangelicals have tended to view cities and urban centers as dens of iniquity and evil. And part of this has been affected by America's racial history and dynamics; even though both suburbs and cities are far more diverse today, there's still a cultural narrative that says that white folks live in the suburbs and other folks live in the cities, and thus suburban white churches have been nervous about urban connections/ministries because of their "otherness."
So when young people say they want to become overseas missionaries, their suburban home churches are likely to laud the decision and support them as their sending churches. It bolsters our sense of identity to reach out in such noble ways, to those "poor natives." But if our young people say that they want to move into a local urban neighborhood or get involved with a ministry in the city, that's not as glamorous or sexy. That makes people nervous. That might be perceived as outside our turf or sphere of responsibility - those city churches can minister to their own.
(The cynical side of me would say that for some predominantly white suburban churches, it's fine to minister to different-looking people on the other side of the world, but not people with different skin color in the same metropolitan area. Somehow the local cross-cultural dynamic is scarier than the exotic overseas journey. I saw something recently about a study showing that multiracial churches tend to work only until kids get to high school age. Then white parents bail out because they don't want their daughters dating black guys.)
Also, there's a dynamic where it's easier to support overseas missions because of the remove and distance. You don't see the daily realities because they're not immediately before you, so it's a safe way of ministering by sending a check or going on an occasional short-term mission trip or hearing updates from the missionaries that you support as your proxies. But local metropolitan urban ministry is nearby enough to be inconveniently disturbing. You're just a few miles away from people who live in contexts far different than your own, and it's troublesome to ask why they live in such unjust conditions when you have your comfortable houses just a half-hour drive away. Better to not get involved at all so you don't have to think about those troubling realities.
I realize I'm caricaturing and overgeneralizing, but I think these are some of the subtle psychological dynamics at work behind why we selectively choose to support some missions/ministries and not others. It's hard for us to be self-critical and ask probing questions about how we prioritize different ministries, since there's almost always good reasons for this or that. So to answer your latter questions, yes, it's all of that - some churches might legitimately feel callings not to focus on local urban work (though this can be a cop-out), and others just aren't aware of the needs or issues involved and need to be better informed. It ultimately might boil down to an idea that "we are not the city, so it's not our problem."
As I've said in various ways, as the suburbs continue to urbanize and diversify, suburban ministry is becoming urban ministry. Suburban churches no longer have the luxury of assuming they can merely stay in their suburban bubbles. It's increasingly important for all churches, urban and suburban alike, to seek the welfare of the whole metropolis. It's got to be a both/and. Limited resources mean that every church needs to be discerning and selective about what they do and don't do, but we can't overlook our own metropolitan area and only deploy resources in our immediate suburban communities or overseas. It ought to be a triple both/and, that we have concentric spheres of ministry that are suburban, urban and global.