I have never booked an airbnb in Portland, but my advice I guess would be to look near the airport, or center your search near there? You may need to stay the first night in one place that just meets your requirement of late checkin, and if you are planning on staying in the middle of the city or something for the remainder of your stay find another place for the rest of the trip that meets all the criteria you are looking for.
My guess is places outside of city/near airport will be used to odd hours or at least have a bit more space to spread out and therefore your late arrival would be less distrubing. It may be the case that a multi-story condo in downtown Portland actually has rules about noise at certain hours and the hosts don't want to be sanctioned, etc. and in the true heart of Portland sometimes figuring out parking and all of that can be a process and your host just may want to avoid being woken at 2 am to go talk through the intricacies of getting into the right spot or finding a good street parking spot etc etc.