If there's one thing Taylor Swift is good at, it's maintaining her public image, and her and her PR people must know that the Swifties are just as divided as the rest of the country wrt Israel-Palestine. Why would she support Biden when she knows half her fanbase will get upset with this decision? Silence has always worked well for her in the past.

She's since dipped her toes into political issues provided they don't damage her image too badly, but supporting Biden right now, especially when she's at probably the peak of her fame and anything she does will draw massive scrutiny and result in hundreds of thinkpieces and "takes," probably just seems too controversial. Maybe that'll change though when the general gets closer and she can frame her support as "anti-Trump" which is one political position she has embraced in the past.