Because Portland is the most accessible place to get to from other places, especially from overseas. There is a small airport in Astoria, but I can imagine the logistics/cost of getting twelve people and their bikes there from the UK would be prohibitive. (And they would probably have to take a connecting flight from Portland or Seattle anyway.)
Yes above sums it up
This is a tour of Oregon not just the coast
I cycled the west coast 19 years ago and the section Vancouver to Astoria isn't as nice as the section we are doing. We did it on a tandem and my wife remembers the traffic.
The route is on the other post I've posted on this subforum for NW.
I do have a little sympathy with airlines. Bikes are tough to handle, X-ray etc so don't mind a small extra charge. I would far prefer they are flown than banned, but please don't hijack this post discussing costings