Originally Posted by
steve64
Some valid possible explanations have already been given.
I'd also point out that you're thought about DEN-SEA being within the aircraft's range is overly simplistic. An aircraft model's published "range" is just a generic number that means nothing when planning a specific flight for the plane.
Leaving DEN, the plane needs to be fueled to fly and make an approach attempt at SEA, then (practically at SEA) divert to an alternate airport. And yoy can't pick any close-by airport as your alternate. The weather forecasts has to meet FAA minimums. Thus, if the "entire northwest" (probably was expecting fog/whatever, then the closest legally available alternate may have been a long ways from SEA. The distance DEN-SEA-Alternate (plus required "reserve" fuel at touchdown and/or anticipated "holding" fuel and/or extra fuel for headwinds and/or zig-zags between storms, etc) may have exceeded the range of the aircraft.
I honestly don't know much about Delta's 717 fleet - I assume they are the basic version and not high-gross weight version. If it's the basic version then you could be right, although it still seems unlikely as headwinds have been very weak on the DEN-SEA routing the past few days and the flight is in the afternoon (i.e., lower probability of fog). If this plane can't regularly make it to SEA with weak headwinds, low DEN temps, and no specifically abnormal WX en route or at the destination then Delta really shouldn't use it on this route.
I expect there is something else at play here - either MX issue as noted above or heavy cargo of some sort (does OP know if the plane was payload optimized)?