LH got subsidies from PDX (waived landing fees) and travel bank support from local businesses. Also, less competition at PDX with no other Europe flights. Portland has over 2 million in the metro area so should be able support the flight.
Also, Adidas North America is based in Portland and has a lot of employees traveling to Germany.
I think it does pretty well, particularly in summer. I took it round trip in May and November and it was very full both legs. Got upgraded from paid business to 1st in May using a united systemwide upgrade.

One interesting thing is that the FA on my May flight said it is the most requested city for flight attendants, so gets a very experienced crew. Probably because it is new destination. They like heading out to the waterfalls and Mt. Hood etc during their layover and get a good rental car rate from Avis.