I thought that American settlers traveling westward in the 19th century were mostly trying to move to the Willamette Valley in Oregon today. You would expect a large population in the state for that reason, plus coastal states generally have high populations anyway.
But something happened so that California and Washington have completely overshadowed Oregon. Even Alaska and Hawaii get more attention. Oregon has the lowest population and population density unless you count Alaska. The state is large, but much of it is sparsely populated except for its west coast, and even then, there's pretty much nothing along the southwestern part of the state.
Meanwhile, California has lots of people paying inflated prices to live anywhere in the state, but especially along the coastline. Oregon has no such hustle and bustle; in fact, it only has one major city, and that city is right next to Washington. Oregon's cost of living is even lower than that of most of the country:
http://taxfoundation.org/blog/real-value-100-each-state-2016
What happened, Oregon?
I believe the most important reason is lack of a good natural harbour, in contrast to Washington or California.
There were various factors behind development and growth of locales on the west coast, such as gold rushes (most famously in California), but one especially important one, especially in the early days, was whether a location was a good base for shipping. The San Francisco bay is a fantastic natural harbour, and San Francisco was the largest city on the west coast until the early 20th century when it was overtaken by Los Angeles, and San Diego bay is also a great harbour. In Washington, Puget sound is also a high quality natural harbour.
Oregon however doesn't have any natural harbours of comparable quality. The mouth of the Columbia river, which might seem like a good option, is in fact highly hazardous for shipping; a sandbar there has caused many shipwrecks.
So in this early and vitally important benefit, Washington and California have Oregon beat, and it's this that is the single biggest disadvantage Oregon had in the early days.
However, today, compared to Washington (7 million pop) at least, Oregon isn't all that far behind in size (4 million pop). And what you said about 'Oregon has the lowest population and population density unless you count Alaska' obviously isn't true. Oregon is 27th of all the states in population and 39th in population density.