My opinion of Jon Ossoff isn't at all related to Sanders' comments regarding his questionable progressive credentials or whatnot; after all, Sanders and OR have made plenty of questionable endorsements in the past. That being said, the fact that the Democratic Party is putting far more resources into a seat like GA-06 as opposed to somewhere like Montana is a testament to how far the party has fallen.
A district that swings 22 points left is called fertile territory. Besides, winning a marginal Trump district like this one means much more to the collective chances of Democrats taking back the House in 2018. Ossoff has also proved himself to be a much better candidate than Quist has, objectively speaking.
Yes, I know Democrats have had success winning statewide in Montana, but the ones who have weren't unabashed left-wingers. Bullock hardly qualifies as one, Schweitzer picked a Republican for his running mate, and let's not get started on Baucus. My point is, the future of the Democratic Party won't be restricted from places like Montana, but it certainly isn't where the base will be.