Personally, I wouldn't mind Oregon joining the B1G. They have been solid in the popular sports in recent history. I just don't think they don't fit what the B1G looks for in programs.
I have noticed market is a HUGE thing for the B1G. With Rutgers, they got NYC and the Jersey area. Syracuse would be better for NYC, but Rutgers works. With Maryland, they got DC and Baltimore. I don't think Portland is quite big enough to be an appealing market, but I could be wrong.
Blue blood is another thing they look for. That was the major appeal for Nebraska. Omaha market is good too, but I think Nebraska's big appeal is them being a blue blood. Right away, the B1G split the conference by the blue bloods (Legends and Leaders). Oregon doesn't fit this either
Then third, academics. Of course, many programs with high academic standards are not very readily available though.
I don't know if Iowa would get in the B1G in recent years. Don't know if Des Moines is a big enough market and Iowa is definitely not a blue blood. Not a top tier academic school either.