Paul Allen is part of the ownership group of the Sounders and the support of the soccer people was a big help in getting QWest/Clink passed and built.
I respectfully disagree with the comment about "least commonality" between Seahawks and Sounders at present, though in the past that may have been true. As the Sounders also play home games at Clink, I would expect there to be a LOT of overlap in the fan base, especially in the people attending games in person at the Clink.
There are only 10 or so Seahawks games out of 365 days of the year. In 2012, there were almost 20 Sounders games at the Clink.
Average attendance at Sounders matches was 43,000+. Total attendance for 2012 was 733,000. The conclusion would be that more people watched Sounders games at Clink in 2012 than Seahawks games. Source: http://www.sounderatheart.com/2012/10/2 ... ndance-mls
The point is there are probably a lot more soccer fans in the Pacific Northwest than you realize.