Via the New York Observer:
The MoMA is one of the first in NYC to officially ban the stick, although it’s technically always been restricted. A representative from the museum stated, “It has long been a policy that visitors may only use handheld devices to take photos, without any camera extension devices. We have simply added selfie-sticks to this policy, which is in place to ensure the safety of our visitors and the Museum’s works of art.” Other museums have either already banned the sticks or are seriously considering it. Gothamist reported that the Guggenheim and The Frick no longer allow them, and that the Met hasn’t yet reached a decision.
The official reason for the ban is the possibility of damage to artwork.