Oh, I don't know, maybe it's because Juneau is a relatively insignificant destination in the grand scheme?...
It's a city of ~50,000 which is more or less landlocked. The airport has a whopping two jetways.
The entire area has but a single Walmart, and it's not 24 hour (an important indicator in my view).
There's some tourism demand (especially when the cruise ships come in), but aside from seasonal fishing and the state government, there's not a whole lot else going on. AS already owns the state business.,
That's why I'm really questioning the choice of aircraft, and how DL plans to fill up a 757.
From a resourcing standpoint, the same crew time could be used flying an additional transcon or an international trip. Instead, they're being used to fly a 2.5hr domestic leg, in a somewhat unproven market that DL hasn't directly served in decades).
They've got ~8500 feet of runway, and it's at 24 ft ASL. There are mountains on all sides, which could present a challenge, but not enough to require the higher thrust to weight ratios of a 757.
That's what says to me that the 757 wasn't the first choice of aircraft to use. The reason for that could be that there's just not enough 737 time available.
The other possibility is DL doesn't want to have to have overwater qualified 737 crews in SEA, or route overwater aircraft there, thus had no choice but to use the 757.
Either way, it doesn't look like the choice of aircraft has anything to do with demand.