She knows which way the party’s moving.
Of course, there's also the fact that Bill Clinton should've been removed from office by the Senate for perjury. In any case, I like Gillibrand and think she's doing great work in the Senate, but 2020 is just not her cycle. What I mean by that is that she is, for a variety of reasons, not a good candidate to run against Trump. She could be President someday, quite possibly even the first female President...but I'm not convinced she can beat Trump. She might be able to pull it off, but we'd be much better off running Sherrod Brown or even someone like Jason Kander (although he obviously won't be the nominee). We could certainly do much worse (Julian Castro, Bernie Sanders, Joe "2016 was your last best chance" Biden, Cuomo or *shudder* Kamela Harris), but I'd much rather run someone who like Brown who would be an ideal candidate to run against someone like Trump. Gillibrand belongs to that group of candidates like Warren and Booker who could win, but would be making things much dicier than they would be with someone like Brown.