Anchorage International Airport has substantial rolling-stock assets and highly trained personnel for snow management.  The snow removal equipment and training are very expensive.  check out the first pic at this link for ANC:  

http://dot.alaska.gov/anc/about/gallery/

And Seattle is much bigger and much more busy than Anchorage.  These assets are expensive and if there is low use, they're standing idle.  Those high costs are then paid for by the traveling public.  It's a benefit-cost trade-off, and would be a bad management decision to be ready for all possible events.  It's called risk management.