why are cities across the river from cities on a state line often

[dead0man]

I've spent the vast majority of my life in 2 of the examples...21 years in the StL area and 17 years in Omaha.  I was on the "wrong" side in St.Louis, though my city (Alton, Franzl is from there too) isn't the train wreck E.Stl is, it's not exactly a nice place.  I have no idea why this happens, but it's probably the "disadvantaged neighborhood" thing Sol mentioned.