Because it's owned by a guy who hasn't been interested in sharing it with his NBA toy.
If Paul Allen had wanted a team, he almost certainly would have gotten one in the early or late 90s expansions. On the whole Allen has showed an antipathy for the NHL - although rumor has it he was kicking the tires in Phoenix at the time Burke sold the franchise to Ellman.
I wouldn't rule Paul Allen or Portland out entirely. While it is true, that other than the very nascent Phoenix talks with Ellman, Paul Allen has not shown any interest in the NHL, it doesn't mean the door is forever closed.
Tod Leiweke, the CEO or Paul Allen's Vulcan Sports empire (Seattle Seahawks, Portland Trail Blazers and Seattle Sounders FC) left Vulcan Sports to become the CEO and part owner of the Tampa Bay Lightning. Who replaced Leiweke? Peter McLoughlin, who was formerly CEO of the St, Louis Blues.
Tod's brother Tim Leiweke is CEO of Anschutz Entertainment Group which not only owns the LA Kings (and is part owner of Allen's Sounders' rival the LA Galaxy of MLS), but is the Arena Manager for Paul Allen's Rose Garden in Portland.
Paul Allen now has multiple personal and professional connections to current and former NHL owners. Certainly his connections to the Leiweke brothers, Anschutz and Peter McLoughlin would be positives in terms of (if not ownership) at least having an NHL tenant for the Rose Garden in Portland.
Edited: Also, Paul Allen is apparently not opposed to sharing the Rose Garden with a hockey team because the Portland Winterhawks play almost half of their home games at the Rose Garden.