I'm not real knowledgeable of the airline business, but one issue is Allegiant typically uses cheaper, smaller airports, other than its base in Vegas. Hence flights into Mesa and Sanford rather than Phoenix and Orlando. Not sure the demand would be there if the flights go into Boulder instead of Denver and then have to bus to Denver International.
On top of that, if you have to pay $220 plus fees round trip to Denver and then another $220 plus fees round trip from Denver to Portland, is it still a good deal?