Not Portland but I lived/drank in Eugene for awhile when Uber/Lyft were still banned (IIRC they've since been let in). Outside of metro areas there is only one answer to that question in my opinion, lots of drunk driving.
In this context it's about mass public transit options and walking distance between a safe residence and wherever "downtown" is. Eugene and adjacent Springfield combine for a population of ~220,000 over ~88 square kilometers with limited late night transportation options other than sketchy and unreliable taxis.
