Housing costs soar as population continues to increase and cities run out of room. Some cities, like Dallas, TX. or Kansas City, MO. remain more affordable as new development just keeps sprawling out.
Other cities, like Seattle, WA. are in regions where sprawling farther out eating up new land is less likely. Environmental rules, such as Washington State's growth Management laws, attempt to curb sprawl plus many of the outlying areas are hard to build on. Steep mountain slopes and so forth.
New development, in cities like Seattle, tend to be in dense downtown like areas. People like these type of areas where lots of urban life is handy and the neighborhoods are walk-able. Problem is, there isn't enough room in these denser areas to accommodate the people moving in. Metro areas, like Seattle, need to devote more of their land to dense "downtown style" development since sprawling out isn't really an option. Everyone who wants to move to Seattle area can't affordably fit in the limited zones that are downtown like; the urban villages. There's still too much land, in their metro areas, that is devoted to single family and low density residential.