This is 100% just from anecdotal experience but I'm guessing the reason is mostly because of the location. A lot of people I've met either don't want to go to school at Dartmouth or don't want to work there because it's in the middle of nowhere. Even their undergrad rankings have gone down. I also heard at some of my interviews in big citys (NYC, DC, LA) that they get quite a bit of money from famous (wealthy) people who live there. Founder of Coca-Cola donated money to Emory, politicians have donated to GWU, hollywood people to LA schools, business people to NYC schools, Zuckerberg donated money to UCSF affiliates, etc. Dartmouth probably doesn't have the same kind of pull because it's in one of the least populated states.
Personally for me the location is a huge plus because housing is super expensive in big cities and I want to minimize debt as much as possible. Maybe Dartmouth could be on a decline but it won't change drastically over just the next four years. I doubt it's anything to worry about