There are two separate issues at play - urban planning and neighborhood development. Portland prides itself on being at the forefront of urban planning, but when many of the roads were created in the perimeter of the city, there wasn't much thought put into it. The city has looked at retrofitting sidewalks into some of these areas, but it's prohibitively expensive.

Oddly enough, a lot of the neighborhoods weren't tightly packed. The lot sizes were decent and the house-to-house spacing was ample. I'm guessing it was more of an issue where, if the builders didn't have to spend money on sidewalks, they didn't.