That also happens with PDX. When BNA tweeted that there were some new routes being announced soon, about half of the twitter replies were guessing PDX, and luckily, they got their wish (albeit, only a less than daily summer seasonal flight, on a less-than mediocre airline).
The same happened when RDU announced new routes from NK. A lot of people wanted PDX, but it didn't happen.
WN will launch BNA-PDX sooner or later. With the apparent focus on making BNA the premier southeastern US connecting point for WN, they'll want this to better connect the southeast to PDX.
If they can only make BNA-SEA work on Sat/Sun only during the summer season (even with competition from both AS and DL). I don't see how they could make BNA-PDX work at all.
WN currently serves LAX, BUR, and ONT nonstop from PDX but not from SEA. There is also more demand for WN service out of PDX than there is out of SEA. While AS has a hub at PDX, none of the US3 carriers have a hub at PDX, and AS's PDX hub is much smaller than its main SEA hub.
There are some destinations in the Southeast that currently have nonstop service out of SEA but not out of PDX, including CHS, FLL, MIA, RDU, and TPA.
In the December 2017 through November 2018 time period, WN carried 3,494,000 domestic passengers out of PDX compared to 3,141,000 domestic passengers out of SEA.
WN might be able to make daily BNA-PDX nonstop service work on at least a seasonal basis since (a) there is more demand for WN service out of PDX than there is out of SEA, (b) none of the US3 carriers have a hub at PDX, and (c) more connecting opportunities to Southeastern destinations that aren't currently served nonstop from PDX would be there if WN adds PDX-BNA nonstop service.